You're probably (definitely) not getting the data speeds you pay for

There are hundreds of reasons why that's the case, but physical limitations to transmission paths is definitely one of the major bottlenecks. Even if the signal is leaving its transmission site (they're not all towers!) at full strength, its rarely getting to your handy at that same level of clarity.

This is because (paraphrasing here):

 According to the Economist, Balakrishnan believes the main problem with current mobile broadband is how phones handle cellular tower switching while on the move. Basically, when phones are connected to one cellular tower, they will wait until the signal is considerably weakened before making the jump to a closer tower.

Yes, there are differences between how GSM, CDMA, LTE, etc handover between towers when moving, but the common ground is that none of them predict where you're going (in this way) in order to start that process before it starts losing packets.

Sources -> http://www.economist.com/blogs/babbage/2011/04/network_fixes?fsrc=scn/tw/te/bl/everymoveyoumake 
http://nms.lcs.mit.edu/papers/index.php?detail=204 via http://gizmodo.com/#!5795321/your-smartphones-accelerometer-could-make-mobile-broadband-faster

There is totally a(n acient) phone in this pic, making it relevant to this post