Capital gains tax calculations

I’ve noted that Sharesight modifies the cost base of a holding when creating the capital gains tax report from the purchase values in the holding details. I found this out when trying to reconcile the differences between Sharesight’s CGT report and one from Class, the reporting system I get from my financial advisor. I can’t find any information in SS about why this happens. The variation in the case I looked at was around 5% lower in the CGT report. If I use the cost base from the holdings’ purchase details, the CGT calculation between SS and Class is the same. Given my financial advisor swears by Class’ calculation accuracy, the question is why does SS do something different?