Issue with Measure not counting correctly

[trevorG1]

Hi I have the following measure set up in my PowerBI report. Counts the number of studies per month. I looked at planned v actual etc. 

(Planned) Mapping Study Started. = Calculate(Count('Raw Data CTU 2023'[Mapping Study (Planned) Start Date]), UseRelationship('Raw Data CTU 2023'[MappingRequired Start(Protocol Pre-Approval Required)], 'Date'[Date]))

I have a the same measure set up for other columns and all reporting the correct count for studies starting per month., but for the Planned Mapping the count is always off by one or two and I can't seem to find out why? e.g. Planned below should be 14 not 13.