The figures you are comparing are Terabyte (TB) to Tebibyte (TiB) - the former being decimal, the latter being binary. Therefore, 48TB raw equates to 43.66TiB, and 33.13TB usable equates to 30.14 TiB.
TB is typically used in marketing/sales, whereas TiB is what you actually get (ie similar to buying a 1TB drive and it being actually 909GB).
All figures shown within the Nimble GUI are the TiB binary figures.
Hope this helps!