I am not sure what you are referring to in your first sentence, I am fully aware of the general rule of thumb differences between brands along with each specific flavouring requiring its own percentage. Thanks for your concern though.
Over here (in the UK) our lemons smell like lemons, I am not sure what they are selling you locally, but you need to find another lemon vendor if you can confuse their smell with vomit.
I knew that the profile of FA Custard was more of a lemony cream flavour before I ordered it, that is what appealed to me in all honesty.
As for FA not changing their custard, I think you may well be wrong on this one.
Their original version had diketones in it (the version I was wanting), but now the custard they sell does not contain diketones, so I am pretty sure that they have changed their ingredients.
This was confirmed by the official UK FA vendor on another forum, he confirmed it in July this year, but I do not know when exactly they reformulated their recipe, nor how many vendors are still carrying the old custard stock.
So, this might explain why you do not get the vomit smell, you may have the old diketone rich custard, or you are not sensitive to it (butyric acid), similar to some people noticing it in the Capella V2 Custard and some saying it tastes just the same as the old one.