Futures long term treasuries v bond ETF returns e.g. TLT or VGLT

[Alan1078]

HI All

I'm new to futures but 've been looking at holding US long term treasuries futures long term as part of a wider portfolio and i cant quite understand the differences in price movement compared to say a LT bond ETF such as TLT or VGLT. When you look at their charts, the price movement seems more magnified with the ETF and the annual returns look very different with the ETF appreciating more over time. 

 

The chart attached shows the current contract Long term US treasuries with the TLT ETF overlayed. Over the 2 year period there is a 6% difference in return. Which doesn't seem a huge amount but some of the price changes during certain periods are hugely different. e.g. june to august 2020, Futures price rose 6%+ but TLT price appreciated over 10%. The figures just seem higher than i would expect from differences in how price is calculated with futures.  I couldnt find any information to explain this and any explanation of futures pricing just seems to complicated to understand at my stage of learning.

When i looked at historical returns for long term bonds, they were significantly higher that what i could calculate using the long term futures price chart. 

Anyway, i'd be very grateful if anyone could point me in the right direction for understanding this.

 

Thank you