There is some variability in noise figure from unit-to-unit.
When components change from one unit to another, this can affect the noise figure. For Example:
Pete Higgins reported on an RCA preamp in TVfool forums
back in August 2013. The left portion of the image below is image is an extract from his image of the circuitry, representing the 1st transistor in the UHF section. Thanks Pete, for your special effort supplying photos and part code numbers.
A unit from Radio Shack was purchased two days ago, the right-hand portion of the image below is a similar portion of the recent UHF circuit.
The location of the date code is shown below.
The PCB has been relaid out with a few changes. Likely most were for productivity and cost reduction (for example; 0603 smd parts are used in the newer board, whereas the older appeared to have 0805 parts).
The first transistor in the UHF preamp was changed from a BFG67 to a BFG520xr. For the newer units, this could have reduced the noise figure by about 0.2 dB.
The BFG67 noise figure at ~20 mA is specified to be:
~1.85 @ 900 MHz
~1.6 @ 500 MHz
Whereas, the BFG520xr is specified at ~20 mA to be:
~1.6 @ 900 MHz
~1.5 @ 500 MHz
This indicates that not all units of a particular preamplifier model will have the same measured noise figure.
This physical difference in units may help support the measurement differences in noise figures found by Calaveras