Archways can be set to be less or more sensitive, (although within a given range, clearly - you can't dial down the sensitivity so much that a threat item is missed). All archway units are tested daily with a standard test piece to ensure that the minimum standard of sensitivity is being met.
If the unit sensitivity is set quite high, your foil pack of chewing gum might be enough to alarm, and yet at another archway it'll pass through.
As ExXB alludes to, they can also be programmed to alarm in a certain percentage of cases randomly, and it's likely this feature that you're experiencing. Airports use this feature to prompt their staff for at least the minimum number of hand searches and / or full body scans required by local regulation. And there is a number... although it differs by regulator, and isn't made public.