Virtually ALL computer fans - case vent or CPU cooler - can be run over a range of speeds up to their max. So the spec they show always tells you the max speed, and the real minimum speed is always zero. But more important is the minimum speed you can set it to run without risking it will stall. Once the fan does stall, it will NOT re-start until its supply Voltage (or PWM signal) is increased significantly to overcome static friction for starting.
That automatic fan speed control system on the headers of your mobo does three things for this. One is to START the fan at boot time at full speed to be sure it does start up, then reduce its speed to what is needed. Another is to limit the minimum speed it sends out so that the fan will never stall. Another is to monitor the fan speed signal and detect a stall if that happens. If it does, the fan is re-started at full speed, then slowed down again. If that attempt to re-start fails, the mobo puts a warning message on your screen to alert you to total failure of that fan. So, knowledge of the minimum speed spec is useful only if you are trying to customize that limit in the configuration of your fan's header.
OP, you are making a common error. You focus on fan SPEED. In fact, what is important is AIR FLOW generated by the fan, but that is really hard to measure. Moreover, the mobo headers do NOT care what the speed is at all - they don't even look at that info to do their control job. Their real focus is TEMPERATURE as measured at a sensor. The CPU fan controls are based on temp inside the CPU chip with a sensor built into the chip. The SYS_FAN or CHA_FAN headers use a different sensor on the motherboard to guide cooling of the entire mobo and case. Each system tell its fan to change speed to whatever it takes to keep the TEMPERATURE at its sensor on target. It does NOT try to control to a speed target or to an air flow target - only looks at measured temperature and manipulates fan signal to achieve that.
What an individual fan does in response to its signal depends on what the fan is designed to do. Given the SAME set of signals from a header, different fans will run at different speeds and generate different air flows. All of that contributes to overall heat removal to achieve the temperature target. IF you connect two or more fans to the SAME signals using a fan Splitter or Hub from one header, they all will respond this way. (You can also get this result if you have fans connected to different headers BUT each header is configured to do exactly the same thing.) Now, IF all the fans are the same, they WILL run at very similar speeds.
You can set up different groups of fans by connecting them to different fan headers and custom setting each header's configuration. But in automatic control mode the ONLY variable they can respond to is a temperature sensor. On some mobos there are extra sensors devoted to particular mobo components, but that's not what you seem to be seeking. There are no mobo fan control systems that allow you to specify that one header will change its output to fans based on the speed of a different fan header.
You appear to be thinking that you can predict from the speed of one fan what the proper speed of a different fan ought to be. That is a form of what's called feed-forward control strategy: predicting a system's needs based on a model with no consideration of actual result. What IS built into a mobo is a feedback system. It uses the RESULT of its fan control decisions on a REAL measurement to decide on any possible adjustment to flow of cooling air produced by the fan. That measured variable is the TEMPERATURE inside the CPU or on the mobo, and that is the REAL aim of the system - to keep the component temperatures in a safe range.
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