The first 24x LO drives were the 4 models based off MTK chipset, but each level added another feature.
iHAS124 = base
iHAS224 = base + LS
iHAS324 = iHAS124 + SE
iHAS424 = iHAS224 + SE
In continuing that trend, LO had to keep making the same models available. However, currently, only the iHAS324 and iHAS424 models are sold as retail packages (at least from what I can tell in the US), and iHAS124 and iHAS224 as bare drives only.
So for the 'Y' series, the 324 is just the retail version of the 124, and 424 the retail version of 224. As Liggy pointed out, the only difference just seems to be the ID string, but they are the same drive respectively. And it comes down to price also, there may not be a need to always buy retail packages with the same software for more $$$ if someone is just replacing a drive, as opposed to installing a drive.