An estimate isn’t some guy pointing at the ground and throwing out a number. An estimate is a calculation based on observable data that yields an approximate result. If they receive new data or refine the calculations, the estimate will become more accurate and change.

They used a new modeling technique with a supercomputer to analyze 20 years of continuous background seismic data to produce the most accurate ‘estimate’ of the magma reservoir ever produced. It’s an estimate because the resolution still isn’t perfect, and it’s an extremely complicated magmatic system. To definitively calculate something, you need to have definitive boundary conditions and that is pretty much impossible with something so huge and complex.

The have a range called the Confidence Interval which has a certain percent likely hood the true value falls in that range (usually between 95-99.5%) so with new data that confidence interval has shifted higher

"Estimate" just means you don't know the exact amount.

you are pouring lemonade into a cup, look away, look back and its overflowing. You estimate there is more lemonade than before You don't have an exact amount other than "more" but you know it's more because the cup wasn't overflowing prior

You can do 4 reps on the bench press at 235lbs. You estimate your 1 rep max is 255lbs. You punched it in formulas to calculate an estimate. Your true 1rm might actually be 256lbs but your estimate is all you really needed to give your 1rm a shot.

In the Yellowstone case its a result of taking readings and analyzing those to get an estimate.