Nets generally have an assigned net frequency. Under normal circumstances, the net control station (NCS) will call the net on the published net frequency. All of the net member stations will set their frequency to the same net frequency and all is well.
There are a few things that will cause the net frequency to vary from the published frequency. The most common occurrence is if there is another station already operating on the published net frequency. The net does not have a right to the net frequency at any time. If there is a QSO going on, the net control station is required to find another nearby frequency on which to hold the net.
Their are other things that can cause the NCS to call the net off of the published net frequency. The NCS may have strong local interference that prevents them from using the published net frequency. The NCS may be having rig problems that cause a frequency shift. The NCS may have accidentally bumped the frequency dial off frequency and not noticed.
In all of these cases, it is the duty of net members to zero beat their frequency to whatever frequency the NCS is operating on. This is the net frequency for that net session. If you don’t hear the net call-up on the stated net frequency, search up and down the band a few kilohertz and try to find the net. When you find the net call-up — that’s the net frequency. The correct net frequency is always the frequency the NCS is on!