I know this is a couple weeks old, but I just saw it, and I wanted to make a couple comments. I can see how, if you were to put a hydrometer into your 5 gallons of liquid (including trub) primary and take a SG reading, it might, might be different than the reading you got if you put it into your post-racked 4.5 gallons secondary.
It seems, however, that you are saying that the actual amount of volume is what is affecting the reading. That is to say, if you took your 4.5 secondary, and racked half of that into a new seperate container, you would get a higher SG reading for the 2.25 gallon liquid. I just want to clarify that, because it must be incorrect.
The reason why none of that matters is because, whether or not you've got a 5 gallon primary with a half gallon of trub, or the 4.5 gallon secondary with no trub, normally, when you take a SG reading, you're just taking a couple ounces and putting them in your measuring vessel, so there is no trub to speak of anyway.
I imagine if you swirled all the trub back into your wort, it would make a huge difference in the SG reading, but otherwise, since it is seperated out, it wouldn't make a difference. Just as pouring clear water onto a dried out mud bed wouldn't change the SG of the water that much. Maybe a little mud would moisten and float into the water, but all the mud at the bottom of the new lake wouldn't affect the SG reading of the water.
Don't want to stir up a hornets' nest, but I just have to make sure I understand what is being said.