"They are not wanting two pounds."
Translation:Chan eil iad ag iarraidh dà not.
Yes, sgillin is the same word as shilling, but the reason for the discrepancy is that Scottish money lost value compared to English money, eventually (by chance) ending up exactly 12 times less meaning that
1 sgillin = 1 shilling Scots = 1 penny English
I have given a detailed answer here.
There is no accepted explanation for the word not meaning a pound of money, but I have put forward a suggestion here.
It is normally used for the GBP (pound Stirling). It is certainly not used for the old pound Scots, for reasons that will be obvious from the link in my other comment on this page. But if you needed a word for some other monetary pound, such as an Egyptian pound, then I am sure you would use this word as there is no other. One exception is the Irish pound that existed until Ireland joined the Euro. I would imagine that the Irish word punt would have been used, just as it was quite commonly in English, since Ireland did not experience the hyperinflation that Scotland did, so the meaning of punt (plural puint) would not have changed over time.