Hi @jiayingpa1 ,
One reason that I could think of is if there are some gated content form conversions that you are tracking but Page ID is not populated in those hits, in this case the Page ID that visitor saw before the gated content form conversions will take credit if Page ID is persisting, assuming that Page ID is an eVar.
If you have any means of verifying on which exact page the conversion happened (using page URL etc.) then you can perform a breakdown with relevant dimension to see if these conversion actually happened on those pages or are they taking credit because of persistence. Another thing you can test is using Page ID eVar's instances metric in the same report which will tell you if that Page ID was tracked on the same hit as the conversion or it persisted from earlier.
Cheers!