Not all these transactions you see are real of course. I'm pretty much sure they have some of their transactions sent to these addresses just to make sure it looks like real but anyone with a common sense would find our that they aren’t real.
I doubt this is the case. Having transactions on a mixer's "deposit here" address does more bad than good to the credibility. but anyone with a common sense would find our that they aren’t real.
It doesn’t make sense to use the same address every time. If anyone check the transactions, they would definitely avoid using this regardless of their legitimacy. The system is used for some of the noobs who don't give a lot of attention to the details. 
