I feel like a broken record!
The ability to "hold a charge" has very little to do with the health
of a battery in terms of its CAPACITY, i.e., the ability to accept and deliver energy at anywhere near its rated quantity.
The only way to know if the batteries have useful capacity remaining is to load test them, preferably at the 20-hour rate. The "industry standard" way these days is to use a(n expensive) testing device like the Midtronics series testers which measure internal resistance and conductance.
If you have six year old T-105's, replace them. Treat the new ones well, i.e., keep them fully charged as much as possible, don't discharge them below about 50% capacity, occasionally equalize them, exercise them, keep the water
level above the plates, add only distilled water, don't allow anything to contaminate the electrolyte, etc., etc.
RE: why one battery in a pair goes before the other, simple matter of math and aging. As they age, batteries deteriorate at differential rates for lots of reasons. In a pair, it's inevitable that one will go before the other.