SAMEPERIODLASTYEAR issue - Inflated value

I have something very odd happening when I'm working with SAMEPERIODLASTYEAR

I have, in Power Query, removed any future dates from my calendar table. My fact table only has facts up to 07/10/2018 (dd-mm-yyyy) so I've limited the calendar to only go up to that date. When I select that very last date in my date range slicer it inflates my LY figures dramatically.

I have a work-around, which is to make my last date in my calendar 08/10/2018 but ideally I'd just want to have it as the very last date that has a fact attributed to it - any ideas as to why this might be happening?

e.g. below shows the issue. If I do my workaround 01/10/2018 - 07/10/2018 gives me 97,435