It's Not the Water Getting In
Well, the waterflow switch is supposed to detect water flowing, and most of the time the "false waterflow alarms" are really caused by water flowing in the pipes. It's not the water flowing into the sprinkler system that is a problem, though; once the water has finished flowing into the pipes, there should be no more room for water, and in theory, more water can't flow into the pipes. Basically, water should never flow again unless there's a fire.
The "false waterflow alarms" occur because that there is air in the pipes. There is air in the sprinkler pipes because usually there is nothing to let the air out after servicing; once the servicing is finished, the air just gets compressed as water is put back in. After the water has gone into the pipes and compressed (or pressurized) the air, in theory, no more water can flow into the pipes.
Because the sprinkler system has pipes going all over the building, totally bleeding the air out of the system is almost impossible, and some air will be left in the system anyway. Can't be helped.
It's the Water Getting Out
The waterflow switch doesn't react as water is flowing backwards; the waterflow switch only reacts as water is coming into the sprinkler system. After water leaves the sprinkler system, false alarms occur as the water is replaced.
The city water pressure is not steady in any city. If there isn't a check-valve to prevent water from going back into the city water system, when the city water pressure goes down, the compressed air in the sprinkler system will push water back into the city pipes. Then when the city water pressure goes back up, the water in the city water system will push water back into the sprinkler system to further compress the air again.
To prevent this water from flowing backward past the waterflow switch, a check valve is used. The check valve is a one-way valve - water goes into the sprinkler system and not back to the city. It holds the sprinkler water pressure at the highest city pressure.
Other Ways of the Water Leaving the System
Most sprinkler systems already have a check valve, but water can leak back through a faulty check valve, or it can be lost out of a leaky drain valve on the system. There can be other ways to lose water in the sprinkler system, but usually these leaks will be causing other damage, and the leaks will be noticed.
Time Delay Problems
There is a time delay mechanism (called a retarder) inside most waterflow switch assemblies. To reduce false alarms, the delay just slows down the reaction time of the waterflow switch. If the switch reacted too fast, someone flushing a toilet so the pressure changes momentarily could set off false alarms. Usually this time delay is set to a time that the fire marshal will accept.
Over time, because the retarder is a rubber air damper, the rubber dries out and the time delay shortens. This could be a problem, but not always.
If there's no check valve in the system now, one installed ahead of any flow switches will prevent many false waterflow alarms. If there's already one installed but it leaks, replacing the valve will solve many problems. Talk to your sprinkler company, they can help.