Difference in Downstream snr

If you measured the SNR using a spectrum analyser you would probably see the same input SNR going into both modems.  A 4 way splitter would enable you to test up to 3 modems simultaneously and have a spectrum analyser connected also.

SNR readings on modems are generated by the software in the modem.  How this is done a matter for the programmer and the method will therefore vary from one modem type to another.  One method is to  assess how much difficulty is occurring in the forward error correction process.  If the input QPSK bit error rate is very low, it will vary from second to second.  For example, a 1 Mbit/s carrier with QPSK BER=0.0001 with only 100 errors per second might have 60 errors in one second and 140 in the next.   The SNR is obviously excellent and high but it is not possible to give a stable estimate value.  The output BER would be virtually zero, say 1 bit error every week or two.

If you are comparing two modems with an input signal of about SNR=10, I would make multiple measurement over a long period and average the results.

At a lower SNR there will be many errors per second and the results will be more accurate and reliable in the short term.

In your case, with readings of 9.8 and 10.4, and assuming QPSK 3/4 rate,  I would ignore these unless they are repeatable and consistent over a long period.

If I was looking to find a technical reason I would suspect power supply connectors, power supply voltages and smoothing capacitors.  Fans, both internal and external, which generate varying magnetic fields and mechanical vibration can be a factor also.  

What matters is the bit error rate at the output (after FEC) near the operating/non-operating threshold of around BER of 10E-6.  Adjust the input SNR to obtain an output BER of 10E-6.  Then measure the input (S+N)/N using a spectrum analyser, convert to Eb/No and compare the result with the modem specification.

If a modem appears faulty, test other modems as well as the problem may be hidden in the carrier itself. Phase noise in the hub modem/upconverter/BUC or in the downlink LNB could be the cause.  There is also the possibility of interference so, if you can, remove the outlink carrier briefly and check that the noise floor below the carrier is clean.

This may help: Eb/No calculator

Example system:

QPSK 1Mbit/s 3/4 turbo FEC
Clear sky  Eb/No=8.5 dB  QPSK BER=0.0001   Output BER < 10E-12
Rain faded Eb/No=4.5 dB  QPSK BER=0.01   Output BER=10E-8

Best regards, Eric.