In case anyone else was wondering about the difference between 'mer' and 'fler', I just looked it up.
'Mer' is used for more of an uncountable thing. Like sauce or sugar. Its the opposite of the English word 'less'.
'Fler' is used for more of a countable thing. Like biscuits or plates. It's the opposite of the English word 'fewer'.
For some reason or other, English has combined these two meanings into the single word 'more', even though it's kept the distinction with the opposite meaning with the words, 'less' and 'fewer'.
As flera means several, the word cannot be used with anything that is uncountable like water. Okay, so fler is then like "several more" or "more to the several". It can be several cups of water but in one cup there is much (not many!) of water. Thanks for this little info. Tack så mycket!
We make a distinction between would like - skulle vilja / want - vill, not because they differ that much but because we find that it's hard to teach both constructions in Swedish otherwise. This is not always perfect and we're discussing whether to keep the distinction in the next tree version or not.
Unfortunately, not all sentences reflect this, and there are such a large number of them that it's hard to track all inconsistencies down. We are, however, getting there - slowly but surely.
I'd put this in the "it's just the way they say it" category. It's used kind of like an intensifier in this context....instead of "Jag vill ha mer sås", you could say "Jag vill ha lite sås till"...a little more sauce. But in other contexts, "till" does NOT mean "more"....it can mean "until", or "towards", or "to", for example. Prepositions are slippery things!