The SPL you referred to is the absolute db value, 0db represents the max value the display can show. The scale on the eq is relative, 0db means flat or no deviation.
Lets say you have a 1000Hz signal and you turn the volume up such that the mic records an 80db signal level. Assuming the RTA max value is set at 90db, you should see -10db (80db is 10db lower than 90db) at 1000Hz showing on the RTA display. At this time -10db equals 0db on the eq because there is no deviation. Now if you raise the eq at 1000Hz to +3db, you should see -7db signal level on the RTA display because you have just added 3db to the signal strength (83db is 7db below 90db).
Lets say you have a 1000Hz signal and you turn the volume up such that the mic records an 80db signal level. Assuming the RTA max value is set at 90db, you should see -10db (80db is 10db lower than 90db) at 1000Hz showing on the RTA display. At this time -10db equals 0db on the eq because there is no deviation. Now if you raise the eq at 1000Hz to +3db, you should see -7db signal level on the RTA display because you have just added 3db to the signal strength (83db is 7db below 90db).