Those USB PIC have a oscillator block that divide anything (well not on all mode but) down to 4MHz, then the internal PLL rise it up to 96Mhz, then you divide it /2 to have your expected 48MHz processor speed.

So why using 20MHz (or else) instead of 4? I no longer see of any good reason apart if, for some reason, you have a bunch of those already in stock.