There's a large difference, unfortunately, but it's the edge cases that the difference lies in. Some professions rely on those edge cases exclusively however.
BUT, having said that,I think there are tons of users that could use Calc for their basic calculations (I worked in a couple of banks and I think probably just 5-10% of users used it as "powerusers" and I suspect less than half of them used functions/code that have no equivalent in Calc)
In fact, my personal opinion is that Libreoffice should abandon PPT and Word - both with adequate online equivalents - and go all out only on Excel.
There is simply no equivalent for Excel on Linux or Mac.
Photoshop + Excel on Linux would be it's killer applications.