Different motherboards have different thicknesses of the PCB as well. Cheaper boards, budget series, are typically thinner. Higher quality boards are thicker. If they go off an average then you could, and we've seen this time and time again in the past, mostly with air coolers but occasionally also with water coolers, end up with a board that is just thin enough for the fasteners to bottom out before everything gets tight between the backplate and standoffs, so that the motherboard is not firmly sandwiched in between. That means when you tighten down the cooler to the standoffs, the mount is going to be loose against the CPU because the hardware it is being attached to is already itself loose.
Putting fiber or hard plastic washers (Small enough around to not reach out and touch anything they shouldn't) between the backplate and motherboard moves the backplate further away from the standoffs, so the fasteners have a chance to actually cinch things down before they bottom out which of course won't allow it to get tight.
Or, you can also take the fasteners you have to a hardware store or fastener specialty shop like Fastenal, and get replacement fasteners that are the same diameter and pitch, but just slightly shorter. If the standoffs themselves are the problem though, this isn't going to work. You MUST have the correct standoffs or else washers or fasteners won't make a difference.