try round it to the same decimal place
[TAXPERUNIT] = Round(1.4,0.1)
I am not sure why this is the case, could be a bug.
I took your field and created a new field with 1.4, did a select and change them to fixed decimal 20.18 to check their precision
The value coming from your field end up being 1.400000000000000133
The one that was entered manually in a formula tool end up being 1.399999999999999911