this topic has been beaten to death. its not just u who experience this little nuance,

it happens because of the shape node that gets automatically seclected after u convert to subD,
and while the shape node is selected and u undo [sometimes*.. 8 out od 10] maya will not delete the extra shape node it created when u undo.

to work around this, i somtimes use this script called polytosubswitcher
or i just be carefull when i model.

heres the script assaign it to a hot key so u can easily go back and forth.