I find that hard to believe, you can calculate all you want, but the lighter arrow will not come out having greater K.E; unless there are greater differences than stated!

1. You did the test with different broadheads, all things must be equal to consider a peneration test.
2. The arrow is clearly shown sticking out of the drum in the entrance pic, and the other picture shows almost a complete pass through one side of the drum; kind of weird.

The thing about Speed Pro arrows, or any arrow too light for the setup; heavier arrows perform better downrange, they have the capability of retaining greater energy at a given distance than a lighter arrow. Also, spine cannot make up for the energy wasted, all that energy was be displaced, and I can guarantee a 300gr Speed Pro arrow is 'soaking' up everything; so where does that extra energy end up at?

435grs @ 262fps: 66.29ft lbs
300grs @ 322fps: 69.06 ft lbs

Even with those calculations, there is still a gap in FPS missing. there is a 135gr difference in those arrow weights, meaning that there should be about a45fps difference, meaning either, that Speed Pro arrow was not traveling at 322fps, or the Axis was faster than stated; or better yet, the test was done without all things being EQUAL.

435grs @ 277fps: 74.10ft lbs
300grs @ 322fps: 69.06ft lbs

Or

435grs @ 262fps: 66.29ft lbs
300grs @ 307fps: 62.77ft lbs

So, I am generally curious as to your DL. I am guessing 28" or 29" DL, and I would like to know how much weight is on your string.

Kind of makes me wonder how this testing was done....