Stitch count

Hi there , new knitter making a sweater that explains the sleeves cast on 38 stitches
Work in k2 p2 rib for 11/2 inches. Switch to stockinette and work 2 additional inches . End wrong side row
Increase 1st each side every 4th row 3 times -38 sts

Work even in stockinette until measures 8 inches

My ? How can you end up with the same # of cast on stitches if you are adding 2 stitches for 3 rows

That does seem like a neat trick. Check that there aren’t different sizes in the pattern and if so, that you’re following the same size throughout. Otherwise, it sounds like a mistake in stitch number.
What is the name of your pattern?