How can it be?
As I know the essence of RM is to be exclusive, to be sold to one customer in the way that a competitor cannot use the same image (and for this reason a RM image cannot be proposed as RF on other microstocks).

That is a common misunderstanding. The core of rights managed is that the customer pays a license fee based on the specific use and he gets a restricted license for a specified period of time.

Royalty free means the clients get a license that is more less free in its use and perpetual. So, an RF license by definition allows the client to do MORE than an RM license. In the classic model when RF came up some 15-20 years ago, it was quite logical to offer the same image at a lower price for a limited use or at a higher price for unlimited use. So RF was more expensive than RM.

This has basically changed over the years, already with the macro agencies in the past but the more so with microstock which was just impractical to offer RM. So microstock almost by definition is RF and that is part of why today RF is considered cheap and RM expensive.

What you are also saying is a potential additional use of RM: The ability to offer exclusivity to a client for a specific use, specific reason, specific timeframe. This can be done only with pure RM images, of course. But it isn't something essential for RM, just a potential additional offer. It is up to each agency and/or photographer to decide if they want to make that additional offer. If that is not an issue, there is no reason keeping you from offering the same image with RM and RF licensing. But once sold as RF, you lose the ability to ever offer exclusivity in the future, of course.