Check the armies per turn:

(Likely armies are stored as the Double type and are prone to rounding errors.)

You're right that they're persisted as doubles and can have rounding errors, but in this case it's not necessarily even a rounding error. It could be explained by correct rounding.

For example, it could be: 99.6 + 57.6 = 157.2. Round all three of those to their nearest integer and it would look like 100 + 58 = 157

Hover over the armies in the top right corner and you can see more precision.

Edited 7/12/2020 18:48:30

Oh, neat! I didn't know about that feature. I'm not sure if hovering over the army camps is supposed to do the same, as I'm not seeing anything. Clicking on the ore/sec does show me a more precise estimate.