You've identified the biggest cause, but there are other factors as well. For example, refreshers are always granted at the end of the year and are only grante

You've identified the biggest cause, but there are other factors as well. For example, refreshers are always granted at the end of the year and are only granted to employees that have been with the company a set amount of time. What this means is that new employees starting in the summer don't get a refresher until they've been with the company for ~18 months. So when their initial grant runs out, there's actually a gap of ~6 months where they are only earning equity from three grants while they would need four grants to fully match the equity they were earning during the prior period.