Is Row 1 marked as a header row? (Inspector -> Format -> Table -> Headers and Footers?
If so, Numbers will always scroll the table such that the header rows are always visible. Therefore, if you're entering data further down the table (e.g. row 20), Numbers will scroll the table so that row 20 (the active row) and the header row(s) are all visible... the inevitable side effect of this is to scroll the first non-header rows up and 'under' the header rows, which sounds like what you're experiencing.
Most of the time this makes sense - it's useful to see the column headers even when you're way down the spreadsheet. If you don't want this, just turn off the Header Rows in the Table inspector.