There are two causes of extra counts:
- Reed switch double closure
- Radio interference (in an analog radio based wireless cyclocomputer)
The source (1) can happen all the time or rarely. If the magnet is incorrectly oriented, it may happen all the time in such great proportions that the speedometer and thus odometer reads incorrect values. In this case, the reading is almost always double of the true reading. If the magnet is nearly correctly oriented, it may happen only when riding at high speed over bumps (due to momentary vibration during reed switch closure), causing a very momentary error practically only in the maximum speed reading of the speedometer and not any odometer errors.
The source (1) can be avoided by using a high quality brand name (example: Sigma) cyclocomputer that is resistant to double closure and by orienting the magnet and the reed switch sensor correctly. See the instructions of the cyclocomputer for the correct gap between the magnet and the reed switch sensor. Be also sure to use the correct magnet! Some cyclocomputers have a weak magnet and others a strong magnet. If you use a weak magnet for a cyclocomputer requiring a strong magnet, you run into problems.
Also be sure to mount the magnet and sensor at the correct height. If the cyclocomputer manual says down near the hub is preferable, do so. If the cyclocomputer manual says high up the fork leg near the rim is preferable, do so.
Cheap cyclocomputers may not have good adjustment options for different spoke - fork leg spacings in different bicycles. At least the Sigma ones I have come with excellent adjustment options, allowing the reed switch sensor surface to be parallel to the spokes.
The source (2) can be avoided by only using digital radio. For example if using Sigma, select STS and not ATS. The source (2) with analog radio (not recommended) can be reduced by having the cyclocomputer and the sensor at the same side, and by minimizing the distance between the cyclocomputer and the sensor (thus mounting the sensor high up in the fork).