See http://avisynth.nl/index.php/Talk:ShowFrameNumber
On reading http://avisynth2.cvs.sourceforge.net/viewvc/avisynth2/avisynth/src/filters/text-overlay.cpp?view=markup#l757 (ShowSMPTE::ShowSMPTE), it looks like the framerate must be an integer (±0.001) or be in drop-frame range (eg, 23.975 - 23.977); framerates like 24.25 will be rounded to 24 and the time calculation will be off.
Should raise an error at least.
Fixed :- http://avisynth2.cvs.sourceforge.net/viewvc/avisynth2/avisynth/src/filters/text-overlay.cpp?r1=1.40&r2=1.41