ETF Index Discrepancies?

Could anyone please explain why some ETFs on trading 212 do not match the index they are supposedly tracking?

For example if you look up IBTS on trading 212 its down 6.2% for the year however, if you look up the same ETF on iShares own website its actually up 4.1% for the year!

also, when comparing different ETFs offered by trading 212 which track the same index there is also a huge difference in performance. For example by comparing IBTA vs IBTS its out by 5% over the previous 3 months alone! Should the performance for like indexes not match each other while the currency and whether its distributing or accumulating be the only differences? I’m not sure if I’m missing something or interpreting this completely wrong. Any help would be much appreciated.

not 100% sure sorry but all I know is the T212 charts seem to match the google finance charts so seem to be correct. someone else can maybe explain the discrepancies better.

Is the iShares chart going back to January and the Trading 212 going back trailing 12 months?

I think it’s this number he is referring to.

maybe ‘discrete’ performance is the key? is this relative to an index maybe…

edit : link to ETF with chart: https://www.ishares.com/uk/individual/en/products/251715/ishares-treasury-bond-13yr-ucits-etf#/