At this point no one will ever know but it sounds like your Dexter steer could have benefited by way of having been finished to a higher degree? A bucket of nuts from time to time or now and again, really won't cut the mustard with finishing an animal.
By contrast, even a small daily ration of beef nuts, say 1kg, over a period of perhaps 90 days, along with good grass could have given you a higher yield of muscle. Hay, will provide maintenance and little else as a rule.
As for the figure of 65% I think there may be some confusion? Some continental cattle ore even some modern beef breeds which have been improved by better selection might achieve 65% as a killing out percentage [ie carcase weight, on the hook, as opposed to the live weight].
I would have thought that many cattle, in particular dairy x or traditional old gp breeds will not kill out much over 50)%.
However, I don't know if 35% of bone would be a fair representation in terms of meat to bone yield; I would be curious to know if in fact that might be the case? logic would suggest that it depends entirely on the quality of conformation and level of finish. Older breeds tended to lay down visible patches of fat at the later stages and few butchers were happy about that.