I'm so confused with this I don't know whether the judge was "not happy" because he believed he should not have been convicted at all or whether he was displeased because he thought the jury should have returned a verdict of guilty to S18.
If the former, then once a guilty verdict is returned the judge is bound to sentence in accordance with the guidelines. Page 8 of this document:
https://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf
suggests that the judge must have considered the offence to be in "Category 2" of seriousness where the "starting point" is 18 months and the sentencing range 12 to 36 months. So clearly the judge could have imposed a lower sentence whilst keeping within the guidelines or he could, at his discretion, sentence outside the guidelines if there is justification to do so. So 21 months indicates that, whatever made him unhappy, he could not have been too unhappy.