This formula uses structured referencing, a syntax that allows table parts to be called out by name. When a table is called with the name only, Excel returns a reference to the data region of the table only. In this case, the entire table range is B4:F104 so Table1 returns the range B5:F105 to the ROWS function.
ROWS then returns a final result of 100. Note that the header row is not included in this count.