Why is 913 d3 considered harder to hit than d2?

Not necessarily harder to hit.  But I think harder to get in the air with the "ideal" launch angle.  That's more so with fairways though, especially when hitting off the deck.  A deep faced FW isn't easy to get enough height for a lot of players including myself out there.

But with the driver, I don't know.  I found the D3 easier and more forgiving.  Actually, for me atleast, whenever a driver head is labeled "tour" or something of that sort where its a slightly smaller head, deeper face, traditional pear shape..etc, I can hit pretty well and always found them more forgiving than the other ones.  But those heads that are suppose to be super forgiving, I tend to struggle with and slice them pretty bad.  The driver market is a weird, weird world.  I find them harder and harder to hit as more and more game improvement features are on them.  Not to mention 46"ish lengths for the "forgiving" models and shorter lengths for "tour" versions when if anything, should be the other way around.

This is my biggest observation with off the rack modern drivers. They are selling you forgiveness and then counter that with 46.5" shaft, but then again, distance is king in the marketing world. Generally, a Tour model will offer a stouter shaft and a less forgiving head. The Tour model may be set open a few degrees and have a smaller volume head and be seen as "more workable".

As far the as the Titleist models, keep in mind that both are being successfully played on Tour and that they are both considered premium equipment. The skinny on the D2 vs D3 is the standard soundbite of the D3 of being "more workable" and the D2 as having "maximum playability". The official reviews contain these kind of generalities, but you will find that unless you try them both side to side, you will not be enlightened because the general forum population is all over the place on preference. Again, two upper tier drivers, but it comes down to which one works best for you.

I'd definitely agree with what both of you posted above. I just bought a new 913 D3 last week, but hit both the D2 and D3 before I purchased mine. Both drivers are stellar and pretty equally forgiving in my opinion. Although, I did find it slightly easier to control my shot with the D3.

I might be wrong but I'm pretty sure the 913 D2 and D3 are the same except for size. Neither is low spin.   It's the 910 D2 and D3 that are different and it is the 910 D3 that is a low spin head.

Deeper faces generally produce less spin.  I think with the 910 and all the confusion with spin vs less spin, they just left it at workable vs playable with the 913.