To answer this question we need to go back to the very primordial beginnings of Equilibrium and how it developed from there. The very first thing that was done, was only a handful of changes fixing the most broken stuff at the time. That's right, EQ started out as a mod with ~4-5 units changed! But it became quickly apparent that work would not stop at just that, and it was decided to create a model and rebuild the game from there. So while EQ did start out and evolved iteratively, this decision does mean that EQ does not have a direct relationship to a specific balance from which it can be traced.
In other words: the way Equilibrium arrived at the stats it currently has are not to do with rounding at all. We used a set of algorithms to calculate every unit stat in the game so that it would be roughly balanced, created a base unit for every type, and then modified them to make each one both unique and resemble the ones in the normal game for the sake of familiarity. EQ was started when a little before 3650 was out, and we used that as a reference point.
That sounds like a mouthful, but in essence this was needed because in EQ the difference between T1/2/3 units is decreased, so the higher tiers dont make the lower ones obsolete as quickly. In order to achieve that, T1 and T3 units needed to be redesigned.
In short, very few units in this game are just rounded numbers. They all contain some sort of balance change.
In the case of ground units, the reason they all do end up with rounded numbers is specifically due to the algorithms used. For example they function in multiples of 80 mass for t2 units, and ranges of 18/23/28/33 (5 difference each). This separates the balancing of units into two layers - the first is basic stats which grounds them into the overall balance context, and the second is tweaks to all those things like rof, firing patterns, muzzle vel, ect to influence its relationship with individual units. This separation gives us more freedom in doing our job, and is our way of isolating variables.
This leads to a few advantages:
1. it becomes feasible to balance the whole game to an ok-ish standard in a short period of time (since then EQ has had >1000 test games to raise it from that ok-ish balance)
2. having round numbers is easier to perform calculations both for us and for anyone playing the game and just looking at the numbers.
3. In the case of mods its possible to add a whole range of new units to the game, and be able to put them in a correct role very quickly.
Of course, this only explains the stats for the ground units, but EQ changes a lot more than that. However, the principle we applied is the same for air and navy too. The algorithms are different for each one, but contain similarities which lets us not only unify each theatre, but also ensure that they work with each other comprehensively.
The next thing is to have a unified buildpower and factory drain model. Our unit stats were balanced with algorithms, so it made sense to apply that to everything else too.
A lot of our units have exactly the same drains, unless they are specifically different in which case its obvious. This leads to no confusion for the player and again, and easier job for us, leading to a better experience for you.
You can see a nice example of this where we actually unround a few numbers because thats what our algorithms predict, and it fits our unified drain model.
Look out for MMLs, Spearheads, Mobile Shields, Decievers. In short support units drain less mass. in a T3 factory that's 14 and not 20/s
Lastly, this has the nice side effect of being easier to understand as a player - everyone knows that mantis cost 50 mass right? wrong! they cost 52 mass and 273 energy.
But no one really cares, they just assume 50m and 270 e and it works out, because its easier to remember than the exact numbers. So now you get a more accurate picture as well. Its a minor effect but something that needed to be explained.