True - Tach time and Real time don't coincide. Tach time runs faster or slower as the engine goes faster or slower. At 2550 RPM (or thereabouts), it's running in real-time.
Thus, Tach time is a good indication of engine usage - the more (and faster) you use the engine, the more (and faster) the components and fluids age.
Some planes have a Hobbs meter that is separate from the tach. It is a real-time clock that runs whenever the master-switch is on or the oil-pressure is up (ie - engine running). A Hobbs meter will run at the same speed regardless of flight performance. Since a Tach runs slower at idle, Tach time typically lags Hobbs time on a plane.
If the plane only has a Tach, then yes, the sum of all the Tach times should equal the airframe time, since that is the only measure available.
If the plane has a Hobbs, then that is typically used to measure airframe time, and will likely not match the tach times.
My plane has only a Tach. Even if I had a Hobbs, I would still use the Tach for all maintenance intervals. It's a truer measure of the workload on the engine and airframe. A Hobbs is more useful for rental billing purposes since it represents the time the plane is 'in use' and unavailable to others.
