We have to remember that pronunciation evolves, and spelling conventions evolve.
When they wrote Mears, they probably intended it to rhyme with Pears, or Mares. "Ea" used to be a normal way of writing the sound in break or steak. Sea was like say, not like see.
Over time, the pronunciation of ea changed to an ee sound in many words, but not all. For instance, we have tear (drop) and also tear (rip).
But the spelling rarely changed. Mostly, people just decided that ea now indicated an ee sound, except where it didn't.
In the Mears case, the old pronunciation stuck, but Mears no longer seemed like the right way to spell it, so people came up with other spellings.