I know my roof/guttering adds 5-15 tds vs straight rain water (depending on the amount of rain), so something unknown is getting in there directly from the roof/guttering, probably harmless enough given the reading but something it being added in the time it takes for the rain to hit the roof and get down my gutters, maybe 1m travel time tops.

Simple things like average rainfall/gutter material/gutter cleanliness can have a pretty big effect on what your roof is adding. Personally if we had the rainfall I'd do a trail run with our gutter water and see what happens, but its a non issue out here since I can not remember the last time it rained.