Those are marked 0 - 20, not 20mhz - as in, it's a range, from 0mhz to 20mhz, and anything in that range is within spec.
So to use 16mhz crystal you need Vcc to be higher than 2.7v I use 3.3v with 16mhz resonator on a 328p.
See page 310 of the datasheet:
"As shown in Figure 29-1, the Maximum Frequency vs. VCC curve is lin-
ear between 1.8V < VCC < 2.7V and between 2.7V < VCC < 4.5V."
10mhz @ 2.7, 20mhz @ 4.5
1mhz per 0.18v
That implies that 16mhz is only within spec at 2.7+6*(0.18)= 2.7+1.08 = 3.78v or higher.
That said, it's widely reported that the chips work at 16mhz @ 3.3v, despite this being out of spec. Might not work over the whole temperature range or something, though.