Posting this here to help anyone who like me had the same problem, but testing the two values were equivalent = TRUE...

Don't forget that when Excel auto populates your formula down a table, as it adjusts the lookup value reference it also updates the target cell references as well unless you have told it not to. The result is that as you go down the table, your lookup range is shifting down cell by cell, increasing the chances that the value you are looking for is now outside the range. You can get around this in various ways by specifying a column name or using the $ reference on the lookup range to prevent them from changing.

Why Microsoft would think that your target range would change when it auto populates a formula down a table is beyond me. At the minimum launch a prompt to ask the user if they want to keep the lookup range fixed and if they say yes, add the helpful $s in for them.