I did a SET DECIMALS TO 2, that was not the problem. The transform error seems to occur only in a compiled .exe, not from the command window. I did another test:

transform(9999999.99,"999999999.99") returns "9999999.99" as expected.
transform(10000000.01,"999999999.99") return "10000000.00" - not expected.

Go figure (literally)!