Curious as to why the definition prompt font and the normal font are different. In many cases, the two fonts have slight differences, or parts missing from the definition font.

Examples are 沉， and 忍. In the first instance, the top line of the 几 is not shown. In the case of 忍 the mark to the left of the 刀 is seen as intersecting the left hand stroke.

Curious if there is any reason behind this or ?

The "normal" writing and tone font is actually not a font, but made of individual strokes we assembled into characters based on looking at a font we have. The definition font is just whatever font you happen to have out of a list we have specified. So there are stylistic differences between our standard font and the one that's displaying on your computer.

If you want a font similar to the one in writing prompts, try installing this guy:
http://www.skritter.com/misc/simkai.zip

I wasn't asking for the "normal" writing mode font, rather the font on the right hand under the definition that appears once you write the character correctly.

I know that some fonts in Chinese are a bit off(not sure why?), missing certain parts adding extra parts. But it seems that in the case of character those fonts probably shouldn't be used. Just had 迹 had an extra mark over the walking radical.

I've got a mac, will I be able to use that font on it? Sorry for being such a stickler ... I know my post/day rate has been a bit higher than I myself would be able to put up with...

I do not know if that font will work on a Mac. The fonts that it's looking for in Chinese are:
KaiTi
楷体
STKaiti
KaiTi_GB2312
Adobe 楷体 Std R
AR PL UKai TW
FangSong
Microsoft YaHei
Microsoft 雅黑
Meiryo
SimSun
Arial Unicode MS
So the first one of those that it finds will be tried, in theory. In practice sometimes it finds one and then uses a default fallback font anyway, probably due to a Mac Flash bug.

I still have it on my list to use the same font stack in the HTML prompt and in the Flash definition/reading prompts. I have to figure out which ones render how big, though, to make sure the sizes aren't whack, so I haven't done it yet.