The 'Pure' conversion is a conversion using the ASA's Equivalent Time Algorithm, rounded to the nearest 100th of a second.
The 'Rounded' conversion is the same thing, but rounded to the nearest 10th of a second. This is the one to use.
The 'Interpolated table conversion' is what you would get if you downloaded the ASA's Equivalent Performance Tables and followed the procedure in this document. This is also what you would get if you use the Pullbuoy site. It's not very accurate because you're rounding a result that is interpolated between two entries in a table that have already been rounded.