(Average Response Time) x (Call Percentage) x (Calls per request)
In this case: 288ms * .36*3.5=383ms
Which translates to 14.6% of 2.62s.
There are a couple of rounding errors that happen if you just take the numbers presented here, the 14% is calculated based on the actual underlying data and then rounded off afterwards.
If you click "Filter service flow" on IMOS, the view will filter down to only show requests that do call IMOS, therefore the call percentage will then rise to 100%. This set of request is then a subset of what you look at here, and thus the contribution will be higher, cause it contributes to every request in that new picture.
I hope this explains it.