"Of all" in Simplified

In Simplified, the phrase "of all" is written as o-o. These are both upright, and it looks similar to a longhand cursive w.

This one surprises me because "all" by itself -- as a brief form -- is written as a o turned sideways. Is there a rule or principle that is hidden or overlooked to explain this o-o combo? I only deciphered it in the exercise because the first time it appears, it's noted in the margin.

Thanks.

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