perhaps a lot of those people hunting on the prairie do use a .243, so the statistics will lean towards a greater number of wounded deer - however, that is completely assumptious of me to say of course, and it may be it's not a common chambering at all and what Muir is seeing is in fact that more longer range shot deer of the medium size do not drop fast enough, or at all, when shot with the .243.
I can only draw a few more potential assumptions and then relate a bit to my own experiences.
first I'd argue that the .243 is more prone to bullet drift than some bullets with higher BC that can perhaps also be driven with more downrange energy. That, 'may' be partly the reason - ie. the wounded deer are due to poor shot placement when 'if' hit correctly the .243 with a proper bullet would put the deer down just fine. In this scenario the shooter picked the wrong calibre given the conditions for sure, not necessarily the distance of the size of the quarry.
I say distance and size of quarry - now here in Scotland (and elsewhere) plenty of people shoot big red stags and hinds with the .243 and a normal soft point up to around 300yds. In my opinion based on quite a bit of experience with the .243 and soft points on reds (so a good match), I argue that even with well placed shots at distances from only 100yds, the .243 and the 100g bullet does NOT put the beast down fast enough. I have seen so many times a deer walk, stagger, run, or just stand there with blood pumping out of it for minutes.
I have more confidence in my .243 than my 30-06 (as I've shot a lot more with the .243 and it's a better gun); however, the 30-06 puts deer down in similar situations a lot faster and more ethically. it's not about allowing for non-well placed shots, perfect H/L shots I'm talking about, identical size deer, identical/similar distances, and over and over the 30-06 performs more satisfactorily. Yes, the .243 kills the deer, and yes, 'dead is dead',,,but, I'd rather spend less time tracking deer, less time watching deer bleed out and suffer, and more time on a following clean gralloch, picture, a dram, and next stalk if time allows.
I would also say I have shot a lot of roe deer with the .243, with both SP's and HP's, and both performed admirably at any distance up to 200yds (I don't see the need in shooting further, at least on my grounds); however, I would also like to point out that even with perfectly placed shots, the majority of runners I've had (although not far) have been from shots within 50 yds.
This leads to bullet choice and speed of course. in the above scenario, clearly the fast .243 bullet whether HP or SP have not expanded heavily and frequently not exited (as confirmed on gralloch in many situations), whereas on longer shots there's been entry and exit with a 1" to 2.5" exit wound. however, push distance too far, or the size of the beasts chest cavity (thus increasing hide thickness and bone structure), and you will probably find that maximum distance for getting a sufficient penetrative effect reducing, as does the 'exit' ability of the bullet (even those with high integrity).
With a larger calibre, closer shots will be delivered with a bigger bullet at slower speeds, so are more likely to not explode on the shoulder, expand, and also exit. on longer shots you should see, due to the calibre, higher momentum and knockdown power, with momentum to break through bone, penetrate to the vitals, and hopefully still exit.
I don't want to talk down the .243 as I love mine and use it frequently on roe deer now, it's a beautifully shooting little rifle, and I see it as the mainstay of my portfolio; however, I think Muir is spot on, we'd be ignorant to close our eyes to the obvious.