why the sender's clock might not in the same pace with the receiver's clock
The problem is not that time is the same, but how you measure time is different.

To follow along with your length analogy, suppose your ruler is slightly shorter on the receiver than it is on the sender. The length hasn't changed, however you might measure/interpret the lengths differently simply because you have a different ruler.

In electronics, often the clock is generated from vibrations of a small tuned crystal. Like everything else in this world, that crystal has manufacturing variations which will effect the frequency at which it oscillates, and consequently your measurement of time will change from one device to another.

You can reduce these variations by using a more accurate time measuring system, but this can get expensive quickly; you wouldn't want to put an atomic clock into every device just so you can essentially eliminate variations in time measurement!

The other way to solve this problem is to design your signalling protocol in such a way that it accounts for these variations. One such protocol is to simply have the sender also send you their clock signal so you can both use the same measurement system.