If you aren't familiar with it, and easy tool to see if things make sense, is volume * gravity but only after the decimal.
4.4 * 87 = 383
5.4 * 68 = 367
They're close to each other so things make sense. The higher volume dilutes what you have and the gravity goes down accordingly.
One thing to note is that volume changes with heat, so you may want to "calibrate" your markings the best you can and then use a calculator to take a stab at what they are at other temps. For example, if your Anvil is correct at room temps, things will read a little high at mash temp (1/16 - 1/8 gallon) because the water has physically expanded. And just off boil, to see where you at, it will be worse, you will read 1/8 - 1/4 gallon high, again because of the water temperature. Let that water cool back to room temp, the extra volume goes away, and you are back to what you expect. The short version is... if you want say 5 gallons in the fermenter, you might need 5-1/4 in the Anvil just off boil, if for no other reason that it's because the water increased in volume with temperature. (Gets worse if the Anvil is off, or you want to plan to leave some trub behind).
Anyhow... if your gravity started low, you have to look at efficiency... how long you mashed, the temperature, the pH, and especially the grain crush.
Also did you use the pipe? If so did you lift and lower it a few times to get that dead space around the sides mixed in? It's pure water around the sides of it that doesn't really become part of the mash unless you physically lift and lower the pipe itself to make it mix in. If you're doing a 5 gallon brew, you can actually have right around a full gallon of water outside the pipe, and it's practically "just water" that comes rushing in at the end of your mash when you take out your grain, causing a ton of dilution.
One other thing, just as a backup plan, is that it's good to have a pound or three of DME on hand. So if you measure yourself to be a bit low, you can make up for it. It's not the same as getting it right, but for some beer might be better than nothing.