Pattern confusion Help!!

Hi I am knitting the James Brett jb616 pattern for boys sweater and I am a fairly experienced knitter. The body part knitted up well but I am knitting the sleeve for a size 26 chest and I have got as far as doing the 89 rows until 55 stitches it is telling me to knit another28 rows and continue until sleeve measures 33 cm but my sleeve is already 33 cm and it is not loose stitches would appreciate help with where it has gone wrong please. Thanks

Welcome to the forum!
Looking at the pattern row gauge (28rows/10cm) 89rows would measure about 31cm. That’s perhaps not counting the rib at the cuff so it seems your close to the measurement that the pattern gauge indicates already. Sounds like you’re right on track. Another 28rows would make the sleeve ~41cm.

If there’s a schematic with the pattern you can check the sleeve length there. From the photo there doesn’t seem to be anything else happening in the sleeve cap, just a drop shoulder.
If possible you might also measure the arm length of the child or of a sweater and account for the drop shoulder in estimating sleeve length.

Thank you for replying so quickly will have a look at your suggestion