The impact of spelt versus rye flours

Hello.. I'm curious to understand "the why" of spelt's impact on a recipe. Last week I made a straightforward field blend bread with rye - it was unbleached white 65%, light rye 25% and whole wheat 10%; 72% hydration; using a unbleached bread flour 100% hydration starter; overnight proofing in a bran crusted basket in the fridge after shaping (about 12 hours) before the bake.  This week I made the exact same recipe with the exact same timing, except I replaced the 25% light rye with spelt.  In both bakes the starter was equally as active. Both were done in a dutch oven and baked (almost) for the same temp and length of time (almost - the spelt was baked about 5/10 minutes less).

But there are three key differences to the breads. First, is the taste - obviously they are different. Second, when I flipped out the dough from the basket onto my bench to score the top there was an obvious difference. The rye loaf was firmer and didn't "open" so much when scored. the spelt loaf immediately looked like it had over-proofed and when scored, the dough opened/spread-out much more - again making me think it was over-proofed. The dough was just very "floppy"..

Here's the first loaf - with rye:

This is the spelt loaf :

Much to my surprise the spelt loaf rose well and had a pretty even crumb - making me think that maybe it wasn't over-proofed. But as you can see from the first picture (sorry, I forgot to take a picture before I cut into it and so pieced it back together as soon as I remembered) the scoring was not as good this time.

But the real difference was the crumb. The rye loaf tastes much better. But the spelt loaf was so much "softer" - in a good way. As I cut it, I had to be careful not to "crush the bread".. the crumb was not too dry or too moist, had good chew, etc. I had made a spelt and oat honey loaf a few weeks ago and thought that the softness of the crumb came from the oats as I hadn't used an oat soaker before, but this was nearly as soft. So I'm assuming from this experience that spelt (a flour I don't normally bake with) has this effect. Very nice crumb. I think next week I'll try a loaf with 50% bread flour, 25% light rye and 25% splet and see how it turns out.

So questions: do you think it was over proofed or is this about how spelt behaves? ; is this unique to spelt, or are there other flours you've used that naturally create a soft crumb as does spelt? ; but the real question is why - what's happening in the dough to cause the softer crumb and over-proofed like floppiness of the part spelt raw dough?

Thanks - bread1965!