FWIW I think the caster affects the wheels "desire" to go straight again. Once a long time ago I had the alignment done on a Celica, and afterwards it would seem to hold whatever degree of turn you made, kind of like what you are describing. I don't know why but I suspected the tires and/or the caster. I don't know if toe-in could affect this. You did the right thing by making the new ones the same as the old. Of course that is an inexact science and also since the old ones were worn it might be hard to compare the bew ones exactly. The suggestion to jack it up and look for binding is good but you might not find any. Maybe the caster is off a bit and the new tighter joints are making that more apparent? Well I am just guessing here of course......

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra