Oh 60 is fast around here too. I mostly meant that I've never seen a download start slow and progressively get faster. If anything it usually bursts at the begihning and then throttles back (comcast did that to me the most)
I've observed download starting slow and getting faster on all connection types: dialup, DSL, fibre, mobile wireless from 2g to 4g. It's just how TCP operates. Interesting that your experience on Comcast is different.
I'm not sure that bursting at the beginning has much to do with the underlying tech but more with marketing.
I wouldn't be surprised if it had something to do with trying to trick speed tests into reading higher speeds before it slows down.
Downloads are expected to speed up over time. Google TCP window scaling to learn more.
It's usually quite common over e.g. 4g. You first have to wake up the radio and attach to a cell tower, which can take 50ms or so. Once you've done that latency for subsequent packets can be much lower (e.g. 5ms).