JPANDRE,
Excel has a precision limit of 15 digits. I wonder what numbers you
have in your lookup table that, after displaying 20+ decimal digits,
you still do not see any difference. Are they all from 0 to 1? Then
this would explain what you say. I have seen "noise" be produced by
rounding functions: you thought you had 2.01 and in reality it was
2.010000000000046.

Are the values in your table array expected to be in the key column of
the lookup table? Are they derived through a rounding function? Are you
using the "exact" version of vlookup? (i.e. VLOOKUP( , , , FALSE). If
not are the numbers in the key column of the lookup table ordered
ascending?

HTH
Kostis Vezerides