Why is my solar panel short circuit rating much higher than in reality?
The I-V (current-voltage) curve for a solar cell is a little complex. Below is an example of the relationship:



Image Source

What that basically says is that the more current you try to draw for a solar panel, the lower its terminal voltage will be. You get the maximum current at short circuit, and the maximum voltage at open circuit.

Solar cells are specified with two values - short circuit current (in your case 100mA), and open circuit voltage (in your case 6V). 100mA does not mean 100mAh, and the rating of the cell is definitely in mA not mAh (mAh would be completely meaningless for solar cells). You may be wondering why it is rated in terms of short-circuit current and open-circuit voltage, well using those two data points you can get a rough idea of the I-V curve for any given irradiance level.

So why do you not get 200mA short circuit current like you expect? Well, the cell generates current as light hits it and transfers energy to electrons which are then free to move as a current. As such it makes sense that higher the intensity of the light applied to the cell, the larger the output current can be. In other words, the short-circuit current is dependent on the light intensity. If you put the cell in a dark room, the short circuit current will be much lower.

You will be seeing only 65mA as you are probably operating at a lower light intensity than when your panel was characterised.



There is another thing to take away from the relationship. If you try and draw the maximum possible current from the cell, it won't actually give you any power - all of the power is dissipated internally over the cell (which is not good for it!). Conversely if you try to run the output at the highest possible voltage, you won't get any power either as there is no current (again the energy from the light source will simply be dissipated in the cell). To get the most power out, you need to operate somewhere around the corner of the I-V curve where the output power is maximal.

There are some good answers on EE.SE about using solar panels and a process called maximum power point tracking.