I have a 21" self-propelled mower although I don't use the SP feature too much to save battery time. This spring as I was mowing a slightly sloping lawn I used SP when going uphill and pushed when going downhill. I quickly noticed that the grass heights were quite different - much higher when using SP. With manual use the grass height certainly wasn't the 3.5" it should have been. Finally I noticed that the whole mower would lift when I engaged the SP lever resulting in the higher grass. Is this normal?