I will be happy with the better measured result than with your calculated estimation.
There are several points to mention with FETs and symmetrical low-noise amplifiers at RF and microwave.
FETs are nice devices in which the internal noise sources ARE correlated, a difference from bipolar transistors. Using this fact, one can couple one internal source against another, and create a COLFET- a device generating (really!) a lower than ambient noise temperature to the input. Some FET amplifiers can be modified to do this. Maybe your balanced LNA allows one side to radiate a "cool" noise into the output to the other.
Measuring low noise amplifiers often require to consider important parameters like noise bandwidth, mainly if the following receiver includes a down-converter mixer. Then if you use a RF band-pass filter before the mixer to cut off one of the RF sidebands, then the real receiver noise figure must be ~3 dB higher than that measured without rejecting one of the side bands.
In measuring protocol, we distinguish DSB (double sideband) and SSB (single-sideband) noise figures if a down converter is included. Typically, NF(ssb) = NF(dsb) +3 dB.