I would guess 21V is near enough. To be honest I have never seen an EPROM that used 21V but 21.5V is very common so perhaps they just shortened the number.
If the programmer uses the correct algorithm, it will keep trying to write to the same address until the data can be read back correctly, if the voltage is slightly low it would just write a few more times before the data verified correctly.
As IanP says, the two devices will function identically, it is only their internal programming control that is different.
Brian.