What factors explain mountain glacier retreat variability?
There is indeed a large variability in glacier behavior, even between neighboring ones. If you look harder, you will even find advancing glaciers right beside others receding quickly. There are multiple factors that can explain such variability, I will list the most important ones:


Orographic control: A climatic change can affect differently neighbouring glaciers. If the wind, air moisture or temperature changes, the corresponding changes in precipitation or solar irradiation can vary on different areas of a mountain massif due to its topography, therefore affecting differently each glacier. A classic and illustrative example is the orographic precipitation illustrated by the figure below. It explains why the windward side of a mountain range could have bigger glaciers than the other. Now, in the case of an increase in -let's say- temperature and air moisture content (that usually come together), the associated  increase in precipitation might benefit snow accumulation in one side more than the other, while the temperature will affect both more or less in the same way. Leading to leeward  glaciers receding faster than windward ones. 

Topographic controls: All glaciers flow downhill, and the speed of such flow depends on the resistance the bedrock, valley walls, moraines, etc. A glacier that is in steady state (i.e. not advancing nor receding) is in balance between snow accumulation in its upper reaches and melting in its lower areas. Even if there is no changes in accumulation or melting, this balance can be broken if the speed of the glacier changes. If the ice is taken faster from its cold head to the warm front, that will increase the melting and more snow would be needed to offset the additional melting.
Most of the resistance to flow is provided by constrictions of the valley walls or protrusions of the bed. As in the figure below (from here), taking a floating ice tongue as example (a.k.a. ice shelf) where most of the resistance to flow comes from the bed protrusion below the "ice rise", in this case such protrusion is called a "pinning point".
Now, going back to your question: Even if two glaciers that are receding at the same rates, eventually one could recede behind a constriction, therefore its speed will increase, pushing it further out of balance, so it will start loosing mass faster than its neighbor and at some point that will show up as a more pronounced shrinking in images like the ones you were studying.
In the case of ice shelves (like the one in the figure), loosing contact with a pinning point can lead to the collapse and breakup of large sections of the floating tongue in very quick and dramatic events.





Reaction time scales: Neighbouring glaciers can have huge differences in mass. And that means that they don't react with the same speed environmental changes. In a very small glacier, you will see advance after a few years of high accumulation. However, in a very large glacier, a few years of high accumulation doesn't make much of a difference. And the glacier can still recede after such events, perhaps due to a few decades of low accumulation that happen a hundred years ago.
Area is not volume: What you see in the images are changes in area not volume. So even if two glaciers are loosing the same amount of mass per year. If one is thinner than the other, you will see that the thicker one shrink less that the other.
Hypsometry: It  refers to the distribution of values of surface elevation of a glacier. For example if two of the glaciers you are observing span from 1,500 to 2,500m of elevation, but one of them have 90% of its surface above 2,000m of elevation, and the other only a 30%. You will observe that a change in temperature that push the average freezing line from 1,800m to 2,000m will affect the second glacier much more than the first, because it will suffer a larger increase in the fraction of its surface exposed to melting temperatures.
Surging glaciers: Some glaciers undergo what is call a glacial surges. They show a cyclical behavior in which they advance very quick over few years and then recede slowly over decades to then start over again. I won't go in detail about this process that is very complex and still poorly understood. But in the images you observe, they might be some variability also to the presences of glacier surges. 


Other factors could be more complex weather redistribution patterns, bed topography, bedrock lithology, basin divide migration, frontal wasting (calving) for lake and marine terminating glaciers, etc.