Understanding how return is calculated against fund performance

Today I took the plunge and opened my first S&S ISA. With it being day 1, it's highlighted a discrepancy between my return and the funds' performance (Vanguard FTSE All world and Vanguard S&P500) that i'd like to understand better and 1 day might be the simplest way to do it!

In the T212 app, it shows the all world fund's performance today as +0.013% but my return is down -0.29%. S&P is down 0.03% and return down 0.43%.

Can anyone explain this discrepancy to a newbie? I tried the wiki page but couldn't find a clear explanation for what this might be. On T212 it said there was no fee - is that just their platform fee and are the discrepancies due to individual fund fees? Online it said Vanguard had reduced fees to 0.19% so that doesn't tally either (couldn't find mention of fund fees on T212 app).

Feeling a bit lost so any steer would be greatly appreciated. Thank you!