Hams' S-meters all start at 40 over 9. Anything less is considered trolling. Never mind that noise is 20 over 9.
Semi-seriously, S-meters are usually bias confirmation posing as science. Even with identically calibrated receivers, different locations and antennas may result in an S7 signal that reads better than an S9+20 signal.
SINPO is based on perceptions and usually close enough for useful feedback. The only problem with the SINPO system for those of us in noisy urban and suburban areas is figuring out whether the fog of manmade noise emitted by our electronic toys is I=interference or N=noise. For example, if we log "I" as high, when we're referring to local RFI/EMI, an op might mistakenly assume someone was deliberately QRMing his broadcast. If we log RFI/EMI as "N", some folks who are accustomed to low-RFI/EMI rural settings may assume we're referring to QRN - atmosopheric noise.
So I usually mention RFI/EMI separately from my SINPO reports. Since I'm in a typical suburban area, mostly using an indoor antenna, RFI/EMI is usually a factor. It can range from an S1 fog of white noise that isn't too bad at all, to S9 snap-crackle-pop that obliterates all but the strongest local MW powerhouses. I've noticed that as more of my neighbors have switched from plasma to LED TVs, the fog of white noise is pretty tolerable now compared with five years ago. I don't often need to tote a portable outdoors to get away from manmade noise.