I am trying to match two criteria in one worksheet to data in another worksheet and then return a value specified in the first worksheet if the two criteria are found in the second worksheet.

Why does

=IF(ISNA(AND(VLOOKUP($J110,'Rpt476 MTD Nov 14'!$X:$X,1,FALSE),VLOOKUP($H110,'Rpt476 MTD Nov 14'!$J:$J,1,FALSE))),0,F110)

give me false positives, but the following does not? What is the purpose of the <>"" in the formula and what is it doing? (I found it on a blog, but now can't find where I found it before and don't know why it's working when the first one isn't working all the time).

=IF(ISNA(AND(VLOOKUP($J110,'Rpt476 MTD Nov 14'!$X:$X,1,FALSE)<>"",VLOOKUP($H110,'Rpt476 MTD Nov 14'!$J:$J,1,FALSE)<>"")),0,F110)