Answer
There could be many reasons as to why this occurs and this has to be analysed on a case-by-case basis.
For example: when observing the IRR metric on the Comps Builder Tool, it is important to remind that IRR of the comp builder is to be understood as an expected return, i.e. a forward-looking measure equivalent to the discount rate on the date of the data. Asset Managers’ IRR are usually realised IRR, which is a backward-looking measure. Expected returns provided by the Comps Builder Tool is the average rate used to discount future cashflows and calibrated to transactions we observable from primary and secondary markets (see methodology here).
Ultimately, the Comps Builder Tool is designed to provide a Valuation Anchor point for investors to benchmark their own asset(s) and thereafter make adjustments where necessary.