The difference is this, and the reason why they can be beaten is this:

Within those horses going around at evens, who win say 45% of the time, there are some who win 45% of the time, some who win 40% or so of the time and some who win 50% of the time. The thing is - they average out at that 45%, so people say "oh, it all comes back to the unwritten law that even money chances win 45% of the time." True, averaged out they do, but if you use your nouse and do the form properly, you can pick which ones are the "true" evens chances (ie win 45% of the time) which are the false ones (ie win 40% or less of the time) and which are the value ones (who win 50% or more of the time). The reason they win 45% on average is because it is ON AVERAGE, just as on average I score about 85 for 18 holes, but it can fluctuate wildly depending on many factors, like course, track condition, distance and sobriety.

The trick is to know which are true, false, and overs. and to bet accordingly.

Duritz.