Why did Philco use different CRTs in same model in 1956?

[Tubejunke]

A fellow member and myself are in the process of swapping some parts. I have a 56 Philco tabletop that employs a 21ZP4B CRT that is on a brightener. I will need a CRT sometime although the set has had a brightener for years. Anyway it turns out that the other donor set employs a 21YP4. I already know the difference between the two. One is magnetic focus and the other is electrostatic. The question is why in the world would they do that? I understand upgrades and improvements but I dont see that as being any big upgrade. Just makes it more difficult from the repair standpoint and swapping parts. It would have been great to go pick up a complete set with everything I need. This difference puts a bit of a damper on the project although there are still other parts there that I need. Someone has a NOS CRT for $100 posted but I am skiddish about sending out a C note and receiving a box of glass fragments. Oh well! I will say this in closing Philco sometimes gets a bad wrap for low quality. At least I read that about Predictas. My Philco has served some 50 years and is just now getting to need attention. The CRT, and like everything else I have needs vertical attention. Whats with vertical circuits anyway. Are they under more stress than everything else in a set or something?