Version numbers are mostly not interpreted as real numbers to a base of ten. In other words, there's no reason that incrementing the 'decimal point part' of 1.9 by 1 would lead to 2.0.

Think of it this way: if that would be the case, we'd already have Perl 6.0 released as stable — i.e. 5.9 => 6.0 instead of 5.9 => 5.10 ;)