Bitstream Vera Sans number characters width problem

[Magnus Kessler <Magnus Kessler gmx net>]

In Bitstream Vera Sans the relative width of the number characters changes at 
different pixel sizes.
I obtain the following with XFree86-4.3.0, libfreetype2-2.1.4rc2 (autohinting) 
screen resolution 133dpi.

At 9px the numbers 4, 9 and 0 are narrower than the others.
At 10px the numbers 1 and 5 are narrower than the rest.
At 11px the number 0 is wider than all others.
At 12px the number 5 is narrower than all others.
At 13px all numbers have the same width.
At 14px the numbers 5 and 8 are narrower than the rest.
At 15px and 16px the number 5 is narrower than all others.
At 17px the numbers 3, 4, and 7 are narrower than the rest.
At 18px the number 7 is narrower than the rest.
At 19px the number 7 is wider than the rest.
At 20px the numbers 1, 4 and 5 are wider than the rest.

and so on...

I can't see a particular pattern here, but in all cases (except the 13px case) 
there are two groups of number characters within which the characters have 
the same width but are different from the other group.

</MK>