Yes, between X and Y, but the OP is suggesting that there be a way to skew the distribution, AIUI.
Unless Toady already has a fixed skew mechanism in place (e.g. 'normal' distribution/bell-curve centred on the range's midpoint) as it is now[1] if a creature has "between 1 and 10 babies, then (say) 30 creatures breeding will tend to distribute their offspring numbers as 3 creatures each. The more breeding creatures, the flatter the distribution (or more bell-like, if that applies), of course, but it'll be random and luck-based whether two (or one, or none) birthings give exactly two offspring and perhaps the "three offspring" cohort takes up the slack. Or the 10-offspring one.
But there'd be good reason to allow up to 10 offspring, but skew it so that pairs are what comes out on average (by mean, median or mode... take your pick according to which measure you prefer). Or, conversely, while as few as a single offspring is possible, usually there's about 8 or 9 of the blighters. Different creatures have different reasons to have different outliers, even with the same absolute range[2]. Perhaps harder for the modder to understand the effects, of course, than "I'll say from 3 to 23, which means it averages out at a baker's dozen of Lesser Spotted Puzuma kits". Unless it defaults sensibly when you miss out the peak-point.
So if we have (with a slightly different tag, to make the old tag still available) [LITTER_RANGE:3:23:] is equivalent to [LITTER_RANGE:3:23:13] is probably equivalent to the traditional [LITTER_SIZE:3:23] but [LITTER_RANGE:3:23:5] means the high potential still has a low average and [LITTER_RANGE:3:23:17] means it could still be as low (within the same range as the former examples), but usually it's fairly high.
I know someone will point out that there's more to the curve than the positioning of the peak, e.g. the sharpness of it, but that could be encompassed as well. [MULTIPLE_LITTER_RARE] could still be useful (forcing a single offspring, or perhaps the lowest number in the given range) but when the "common" situation does not apply the rarer one can be skewed left, right or centre...
e.g. if modding dwarfs accordingly, when Urist McWife doesn't produce the usual single sprog, she's actually most likely to get quintruplets... I could easily see that modded in by the player to make for nice surprises for the player (not to mention the mother herself).
[1] And ignoring [MULTIPLE_LITTER_RARE] as an already existing skew towards the 1 offspring, regardless of the range
[2] Actually, I'd go the whole hog and go for a mathematical description of the curve, affecting where the curve is centred and how it is skewed and how flat/precipitous that skew might be. If that was augmented by a limit of physical impossibility (e.g. a cap applied so that the one in a million chance arising out of the maths concerned that 20 offspring were produced from a nominally 5-6 offspring creature was reduced to 10), and naturally if the equation popped out a value lower than zero then it would be zero itself, i.e. a failed pregnancy), then we could even give the creatures heritable traits for marginally larger/smaller litters than their (but significantly larger litters linked to smaller/weaker individuals, as a natural balance) as well as environmental effects (pregnant creature that is hungry for a key part of their pregnancy has a further reduced litter size[3]). But that'd take more maths and calculations. Not sure if it's in Toady's vision.
[3] For now, ignoring the actual real-world situation that women whose mothers suffered starvation during pregnancy can be seen to have smaller children themselves (i.e. the grandchildren of the ones who suffered).