Same Specs, different speed? WHY?

The only way that you could have "EXACTLY" the same software, is to boot both systems from the same volume.
Once you begin using a cloned system, regardless of how perfect that image might be as a source, on other hardware, it becomes (slightly) different.
Not likely that you would get a precise repeat of a timed process like video conversion, etc. There's hardware tolerances in the timing of memory, the memory bus, the processor, system bus for access to storage that would all come in to play here.
Could be that everything adds up on the faster one, to end up with a faster task. And, the other system simply adds up on the slower end of tolerances. But, the two might reverse position on some different task, with a different mix of processor, storage, and memory use.
It's really the same as running some random benchmark app on both Macbook pros. No benchmark will give you the identical results twice in a row, other than within some set of tolerances. MacBook Pro "A", specced the same as MacBook Pro "B", will not necessarily test and benchmark the same, other than within a range of values. All you have with two "identical" computers, is to know that both have the same hardware, and the same set of software. Beyond that, they will not be identical to each other. I expect that you have one MBPro at the high end of tolerances, and the other is more to the low end. So, both good - but you don't get to expect them to perform identically in real life.

That being said - if you normally show that same 30% difference, that seems like a lot. I would expect manufacturing tolerances might allow for about 15% (I don't have knowledge about those engineering tolerances, but that seems reasonable, where 30% on the same hardware might be too far off.