It's sticky because as time goes on, the $10 will approach, in worth, the $2.13 (or whatever base now is) of today.
So, either the base of $10 has meaning or it doesn't. It's meant to be a minimum, so the servers will be guaranteed at least $10 per hour, whether or not the restaurant is busy, whether or not a server gets some low-ordering tables, etc. But as the $10 becomes worth less over time, it will just become like today's minimum wage which was set so many years ago.
So if the 20% is always going to come into play, then why bother invoking any minimum? But if there is actually a need to invoke a minimum, then that minimum needs to keep up with inflation.