Got a spool of hook up wire in your shop? Is the inside end of the wire accessible? Measure the resistance of this reel of wire. It won't be zero. A transformer is a coil of wire around an iron core. The wire has resistance. That is what you measured.
Transformers function by transforming between IMPEDANCES, not resistances. You measured resistance, not impedance.
Transformers can be uneven between plate leads due to their construction. There are two ways to wind one with a center tap. One way is to use two pieces of wire and wrap them around the core together. Each piece becomes half a winding. We join one end of each to make the center tap. because the two wires are the same length, they will measure the same resistance. Oh there might be a small difference, but it won't matter to the circuit. The other way is to wind one long piece of wire around the core. When you get to half the turns, you connect a center tap wire to the winding wire, then continue to wind the rest of the wire. That results in one side being tight around the core, while the other half is wrapped around the first. The number of turns will be the same for both sides, which is what counts. But the outer windings will be longer as they have to go around a wider wrap, so the second half being longer wire will have a larger resistance.
If I recall, the little Peavey Classic 30 output transformer had one side at about 90 ohms and the second side at about 120 ohms. Perfectly normal..Uneven resistance will make for uneven idle current, but the turns ratio is not affected, so the thing works for audio just fine. Your 316 versus 450 is a very similar difference in terms of percentage.
This has little effect on tone. Consider the impedance of a transformer might be 6000 ohms, having the two halves differing by a few tens of ohms will not make a difference.
Fir a given tube current yes idle current will be a little different. if it bugs you adjust for the average between them, as in if we get 36ma and 40ma, consider it as 38ma for both. Remember this is just a guitar amp, nothing precision here.
To the tubes, this difference is miniscule.