With a Match_Type of 0, you're looking for exact matches. Might there
be unseen differences in the data that appear to match?

This could happen due to rounding; for example, a lookup value of 2.5
would appear to match a cell that reads "2.5" in your lookup range.
But if that "matching" cell is formatted to show a single digit and the
value is actually 2.48, there is not actually an exact match.