Calculation Difference From Same Data

Hi All,

Hoping someone can help me with this, it's more of a math issue but I'm just lost. I have some data from equipment downtime which i use to calculate Technical Availibility %. each equipment has its own TA%, i use the sum of these values to get a TOTAL downtime for all systems and use this figure to get a % in this case 98.10%. Using the average of all systems is get 98.30%. I'm sure there is a reasonable explanation for this but if someone could take a look at the attached sheet for some insight it would be greatly appreciated.

Thanks

Thank you AliGW, i kind of guessed that it was a rational reason! Thank you, you've actually made alot of peoples work life easier :D

i was just looking for that! thanks