Unfortunately, you are in a minority. Other than minute differences between each unit. I don't know what else it could be when you are OTA only. CATV is another animal.
Unless it's the amount of recording, scheduling and/or use. A few random programs, sprawled out over a day vs numerous overlapping recordings on both tuners in a short period of time I bet would make a difference.
Question, has anyone timed the time it takes to delete recordings? That was something that I can't understand why it takes so long unless the system is actually erasing the recording itself instead of just removing reference to it like Windows does.
Free, abundant OTA television separates this country from many others. ATSC1 has only been in force since 2009. The wireless industry has enough spectrum. Enough of 'planned obsolesce'.