I usually put the location marker in by typing the grid reference, which I’ve worked out beforehand. I’m finding iSpot nearly always then places it a few hundred metres away. Is it clicking it to some invisible lat/long grid line?
I am using up to 12 fig refs here https://www.ispotnature.org/communities/uk-and-ireland/view/project/746155/100-x-100-x-100-orkney - I don’t have issues with accuracy but I sometimes get the precise reference via Geograph https://www.geograph.org.uk/photo/5569328 - click on the map and use the zoom - cursor supplies a 10 fig ref.
If you are using 4 fig. I think it goes to the bottom left corner and if you have the Hide precise location box ticked then it will be a very wide area. I have never had an issue from a six fig OS Ref.