I noticed something strange in the parshe of Tetzaveh regards the verse of Shemot 27:20 in which HaShem commands Moshe saying: "And thou shalt command the children of Israel, that they bring unto thee pure olive oil beaten for the light, to cause a lamp to burn continually" And it's the meaning of Tetzaveh itself. Most often it's translated as 'thou shalt command' as in the OJPS, but it actually has a different grammatical meaning, the NJPS translates it as 'You shall further instruct' which comes closer to it's true meaning. In this case it seems to mean that Moshe will command the people of Israel, somewhere in the future, about the preparation of the oil. If it was meant that Moshe should immediately instruct the people of Israel it should have said "Command the Israelites" as is indeed the case in Sefer Vayikra (24:1) in which HaShem instructs Moshe to bring pure olive oil.
What's the reason that HaShem commands Moshe beforehand? And what does it teach?