The safest bet to select entire worksheet is through pressing CTRL+A three times not two times or one time, if you are using shortcut (The safest bet is to press the triangle between 1 and A as marked in Red in the given picture. The tip is for CTRL+A shortcut.)
This peculiarity of CTRL+A shortcut comes into picture when your worksheet contains tables also. Suppose, you have a worksheet like below which has at least one table.
1. Select a cell outside any data range, press CTRL+A and it will select entire sheet.
2. Select a cell inside A1:D2, press CTRL+A once and it will select A1:D2 range only. Pressing CTRL+A again will select entire sheet.
3. Select a cell inside header row in the table , press CTRL+A once and it will select entire table. Pressing CTRL+A again will select entire sheet.
4. Select a cell inside the table data range i.e. A6:D7, press CTRL+A once and it will select table data range i.e. A6:D7 only. Press CTRL+A again, now it will select entire table i.e. A5:D7 only not entire worksheet.. Now, press CTRL+A again i.e. 3rd time and now, it will select entire worksheet.