I'm not sure why your fuel consumption is changing, but I've got some possible leads for you.
First, though, some details for ya!
1- your tank is going to be rated as maximum fill point by the manufacturer- that means the total volume. But part of that volume is above the filler hole, so that's right out. And of course you aren't going to fill it up all the way to the filler anyhow... and you probably have some space below the petcock in the bottom where all the tank sludge gathers... so not a full tank, ever.
2- petcocks! These bikes have a three-position vacuum petcock. One way is run, the other reserve, and prime in between. It's set up this way so that when you're running along and it sputters you can just flip it completely around and don't have to worry about which way to go (if it's in the middle) or fumble around finding the middle position (if you're going to the middle) or anything.
It's a vacuum operated cock, which means that you've got a vacuum tube running from your carbs to your cock (hush!) which opens the cock when there's a vacuum on the other end of the tube (yes, the cock won't work unless there's something sucking on it... everyone get the jokes worked out). No suction- no fuel. This way, when the bike stops, the fuel supply is automatically turned off.
Jutting up inside the tank is the standpipe, which has a hole at the top and a hole at the base. On "run", it's taking fuel from the top of the standpipe, and when the fuel level drops down below that, it sputters and dies... you flip it over, it starts taking fuel from the base (an inch or so lower down) and boom! You've got another half gallon or gallon or whatever it is.
That's RUN and RES. PRI (prime) is the same as RUN, but it's designed to prime the carbs if they're empty. If you've just installed clean carbs, or if you've drained them for the winter or to get water out or whatever, you'll need to prime those babies 'cause the bike won't start without fuel, and the vacuum petcock won't turn on 'till it's running. So you turn it to prime, which opens the cock without suction (yes, that can happen!), and your carbs fill up 'till the floats rise, the needle valve shuts off, and the fuel supply cuts out. Now you're primed, you set it to RUN, thumb that starter and rumble on out.
So, why not just run it on PRI all the time? Eh, no reason really, but if you forget to turn the fuel off (turn to RUN, since these bikes don't have OFF), and your floats stick, or the needle valve has an issue, or you gotta leaky carb, and now you've got gas trickling all over your crank case. Not a good scene.
So now the question becomes "Why isn't my bike running on RUN or RES?" Well, all three use the same fuel lines, so it ain't that... all three let fuel run through at about the same rate, so it ain't that either... RUN and PRI use the same source (top of the stand pipe), so it ain't THAT... it's gotta be your vacuum valve. I'll bet you dollars to donuts your vacuum hose is leaky or just plain disconnected.
Now, fuel consumption! How's your power? Are you running lean or rich? Did your idle screw get bumped and you've got a fast idle? All those things could affect your fuel rating, and course you might be putting different amounts of fuel in there- are you tracking by tank or by how much you're putting in per fill up?