This is an old problem caused by a difference in how the two applications store floating point numbers.
Both applications store them in a binary form of scientific notation, but the number of "significant digits" used by each are not the same and this will result in what you are encountering after importing your data.
The fix is to round the data after import to a very large number of decimal places using the round function in either a looping script of via Replace Field Contents using the calculation option.
see here for more info:
The other option that will fix this is to import from a text file, i.e., a CSV file. So, if it is feasible for you to save your Excel file as a CSV, give it a try.
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