How do they replace cracked iPhone screens so quickly?

I cracked the screen on my 4 the other day. Because I live in NYC it’s easy to pop over to Chinatown and get it replaced in 20–30 minutes for $60. But how is it that they’re able to do this extremely involved procedure in such a short amount of time? I looked up several do-it-yourself videos and directions online, and it seems that for even the savviest of techies it takes about an hour an a half, because one must take all the components out of the casing. Is it really just that these guys in the shops are so practiced at this that they can be that fast and do a good job (these places have loads of great Yelp reviews), or is something fishy going on here?