Astronomy

For large estimates, it would be suspicious if it wasn't round.

The number is 40,000,000,000,000,000,000. That can mean two different things.

1. It's exactly that many. Not ,001, not ,999. That is your "assumption".
2. Not all of those digits are significant digits.

To illustrate with an example of that article:

if a length measurement yields 114.8 mm, using a ruler with the smallest interval between marks at 1 mm, the first three digits (1, 1, and 4, representing 114 mm) are certain and constitute significant figures.

Let's assume they measured these 40 quintillion with a "ruler" which has a resolution of 1 quintillion. In that case, they could just as well say the number is 40.1539577 quintillion, or dream up any other combination of digits after the leading '40' (like, for example "000,000,000..."). Because they don't know.

But if they noted a non-zero string of digits, readers would wrongly assume their ruler has sufficient precision to measure these smaller digits.

So this notation conveys two insights:

1. We know the first digit(s): It's 4. (and maybe 40, 400, ...)
2. We don't know the smaller digits, but we do know the magnitude.

So a non-round number would be suspicious, because it pretends to have precision which it most certainly cannot have.