Why can an effect size of 0.40 be gained in a shorter timeframe?

There are a few reasons:

• Teachers often chunk curriculum and teach it in bursts. Students usually only get exposure once to a topic. The assessments the students receive is related to these chunks. This means that d=0.40 is appropriate for shorter timeframes covering material students will only see once a year.
• Often the difficulty of the assessment(s) reflects the nature of material covered. This means that progress of 0.40 can be expected.