Cursoring above the first line has an odd effect

I'm using a bluetooth keyboard. If on the first line of a document I cursor-key upwards, the cursor does not move (naturally) and the displayed line number stays as one, as expected. However, it appears to count however many lines before the start I up-arrow to, since if I di it 7 times, I then need to press 'down' 7    times before the cursor starts moving again!