@Patrick-Murphy Having looked at some of these, it looks like they're due to gaps in the prices. So basically short amounts of time where we weren't able to capture price snapshots, so your exits have either been missed completely or recorded later than they should have been.
These gaps can be due to latency or brief outages of the Betfair API and occasionally due to some issue our end, like the server issue I posted about recently here.
We regularly plug these gaps where we can using the Betfair historical data, and we've recently improved our audit scripts to better identify when they happen, so have been going through the whole database again to apply this improved method. It's a fairly big task but should be finished by the end of next week.
Sometimes when the issue is at Betfair's end, the data doesn't even appear in their historical data, which means we won't be able to plug the gaps at all. This will represent a fraction of a percent of trades, but I can understand that if you're backing at very high odds and large stakes, it could skew the profit figure.
Maybe we need a way of marking these in the database. We could also add some kind of "report and exclude" button to each individual trade in the results table, so that if you spot any of these yourselves, you can submit them for review and make them not appear in your backtesting results.
I'll finish the DB audit and then see what we can do to deal with any issues that still remain.