Interpolate between data - NOT WORKING?

I haven't got time to go through it in detail. One observation: In 1 and 2, you have "grade" sorted low to high. In 3, you appear to have grade sorted high to low, but you have not adjusted the 3rd argument of the MATCH() function to account for this.

It's kind of hard for me to debug other's work, because I don't have the newer versions of Excel. A few suggestions that may help you learn how to debug these things yourself:

0) I'm sure the regulars around here get tired of me suggesting this, but, if you are not married to Excel, QuattroPro (and maybe other spreadsheets) has a built in linear interpolation (@LINTERP) function. It might make this problem a lot easier to solve.
1) Probably the first thing I would do would be to call the "formula evaluation" tool off of whatever passes for the formula auditing menu in the newer version. When you have several functions nested like this (match within offset within forecast), this tool will allow you to take one incidence of the function, step through the formula to see a) if the match part of the function is returning the expected result and b) if the offset part of the formula is returning the expected result). Gut feel on this problem is that either match or offset is returning something unexpected, which is causing incorrect results.
2) It seems that our goal on forums like this is to cram as many nested functions into a single cell as we can. I tend to buck that trend, especially when debugging multiple nested functions like this. Put the match part of the function in its own cell, then put the offset part of the function in its own array, then finally put the forecast function in its own cell. This allows me to see each part of the function separately, which makes it a lot easier to see where the overall formula is going wrong.
As noted, I suspect that, for some reason, your offset/match combination is not returning the right range for a given input value, which is why you are getting erroneous results.