Why do you italicized the volume number but not the issue number?
Answer
Only the Gods of APA can answer this question ... ;)
My best guess would be to highlight the difference of what a volume and an issue represent. A volume is generally a year of publication, i.e. volume 21 would be the 21st year the journal is published. Issues are within each volume year, and the number depends on whether it the journal is published monthly, or quarterly, etc.
You could argue that having the issue in parentheses would be enough - 21 (4) - we hope that they style committee at APA had good reason for what they decided! Our job is to just follow the rules.