inSSIDer v3 reports differently to v5

inSSIDer v3.1.2.1 reports a particular wifi connection max rate at 54 but when I run inSSIDer v5.4.0.0 on the same computer ( without moving ) it reports a max rate of 1020.8 for the same connection.

How can this be and which one will be correct?

inSSIDer v3.1.2.1 also reports max rate of 300 on the same connection on another ( very old ) computer and it used to report 450 on the same connection.

Thank you