lower SNRM can indicate a lower target margin but the thing is the modem displays SNRM not SNR so need to look for signs whether it was an SNR change or target change
or you synced at that speed with 9db target , as you know the attainable calculation is sketchy
i assume the SNRM is indicative of how much is reserved so given your very low attenuation [11db] i suppose you were able to sync at that speed with 9db target
edit: so higher target margin is better, if someone can cap his connection at it: so, if your target was indeed 9db and reduced to 6db that could indeed mean your line fared worse [less SNR] & needed 6db to maintain the sync speed.