Hello
I think maybe you counted the number of stitches in the repeat incorrectly.
87 - 3 = 84, yes, then:
K1, (yfwd doesn’t use a stitch but creates 1) = 1
S1, k1, psso (psso does not use a stitch it decreases 1) = 2
K1 = 1
K2tog, yfwd (a decrease and an increase) = 2
So 6 stitches are used during this repeat
84 ÷ 6 = 14
As the decreases are all balanced out with increases (yfwd creates a stitch) the total number of stitches at the end of the row should equal what you started with.
Row 3 adds lots of stitches so you either need to NOT count stitches this row or work out how many it should be with that number of yfwds so that you can check on your progress at the end of the row.
Row 5 has more decreases than it does increases, I have not worked out the maths but it looks very similar to the lace top I recently completed where row 5 reduced the number of stitches back down to the original count (for you 87).
My pattern told me in the notes not to count stitches at the end of row 3, perhaps yours has a similar note?
I found it better to work out how many there should be at the end of row 3 so I could check my work each row.
I hope this helps. Please do say if you need more help.