Generally lambs put on and lose numbers with feed / lack of feed, the letters are the conformation and that's more to do with breeding.
However, I would have expected a Charollais x Shetland to be R at least - O is rather poor, the sort of thing you'd expect for pure primitive or something like a pure Swaledale.
I have been told that you can drop a conformation class by not feeding just a little cake (1/2lb per head per day for a commercial type) once autumn kicks in. And it's a very fine lamb indeed that can make the Holy Grail E2 or E3L (top grade) with no cake at all on our poor northern England hill land.
If you had deadweight in excess of 21kgs and graded O3L then these lambs were simply too old - grown too large in the frame. At that size, then yes you'd have been in the 'heavy lamb' (less money) bracket had they not been quite lean.
Top money is for anything E, U or R 2 or 3L and
not more than 21kgs deadweight.
So yes, the art of producing finished lambs is to get them away at the optimum deadweight / grade. That differs according to breeding and feeding and takes some experience to get right. With your very different types of lambs then yes, to achieve top dollar you will need to make sure each goes at the optimum time for that type.
It's a hard job with only a few sheep; we 'draw' lambs from each batch about once a month, sometimes once every 3 weeks, throughout the summer. All kinds of factors make lambs with similar breeding and feeding finish at different times. Gender, whether entire or castrated, whether it was a single, twin or triplet, how much milk and what quality the mother had, whether or not it ever got wormy or flukey, had lungworm, orf, or any of the other myriad of infections and diseases there are around on any farm... Whether it was ever lame and reticent to move about therefore...
So don't beat yourself up if you can't get similar lambs to finish at the same grade at the same time! None of us can, it's just that a larger farm with enough numbers can keep finding collections of 20 to send away every week, or even two or three times a week, for a couple of months!