Or HaChaim on Numbers 11:24/אור החיים על במדבר י״א:כ״ד

1 ויצא משה וידבר אל העם, Moses went out and spoke to the people, etc. He first spoke to the people alone, and then he summoned the newly appointed elders. This did not correspond to the order G'd had commanded him in verse 16. There he was told to first assemble the prospective elders at the entrance of the Tabernacle and to speak to the people only after these elders had been granted a measure of Holy Spirit from G'd (compare verses 17-18). Moses assumed that it was G'd's intention that he should inform the people about what was to take place. If G'd had intended to appoint the elders first, why would He have told him in verse 18 to address the people?

2The reason G'd wanted the people informed before the prospective elders were assembled was that this formed G'd's answer to Moses' complaint that he could not carry the burden of leadership of the people single-handedly. First G'd replied to Moses' complaint; afterwards he told him to address the people. G'd knew that Moses was intelligent enough to read His mind concerning the sequence, as indeed he did.

3It is also possible that the words: "Moses spoke to the people the words of G'd," referred only to the instruction to gather the elders. Moses thought he would deal with that subject first. He would then speak to the people again telling them to prepare for the morrow when G'd would provide meat. The latter explanation appears to be in line with Midrash Tanchuma.