Why do these identical weapon coils give different results?

This has been observed a handful of times by users and the general consensus is that there are hidden decimals that you can't see but will affect the outcome of applying the mods. From this thread:

Two mods with the same x% stat don't necessarily increase by the same amount. I think there's some kind of decimal system in place.

And this thread:

I'm fairly certain that there is decimals that you don't see.

Like you said I've had multiple say 10% ones. Normally they'd add like 5 but sometimes the bar will move up and it be 6 or 4.

The damage/modifiers increase is based on how full those little slashes are.

And from this thread:

The 20% mod changing to 19% could be a rounding difference. I've always known that mods have decimal numbers that you can't see. 2 identical looking mods can occasionally give different stat bonuses, and it's because of the decimal.

In the last thread, users were discussing an update that was released after the time this question was posted that changed a lot of their mod values. It may be that the invisible decimal was a bug and these values have been changed to more accurately represent the boosts they grant, but unless you still have the mods from the question, it will be hard to prove, though I suspect that this was just another one of these cases.