When the counterfeit manufacturers start copying a product they start out making exact copies even making them as good as the original, they then set about reducing the quality of the parts and manufacturing until they get to the point that they last for 12 months and no more, that then becomes their standard for manufacturing.
When Japan started making electronics they were the worst in the world, they achieved sales but soon realised that they needed to improve quality or they would not sell product, they not only improved their standards but set new highs for standards and now Japanese electronics is considered the best.
Malaysia and other countries in that region are at the point where they know they need to lift their game or lose sales to China.
China is at just past the start of the cycle, they are at the point of reducing quality until they can manufacture it the cheapest, people are starting to realise that Chinese products are crap and at some point China will have to lift their game or some other country like India will start the copy cycle.
The metal detectors being counterfeited would likely have circuit boards and parts that are identical to the genuine ones.
Some manufacturers will have substituted the internals for some that will give the appearance of working properly and will even detect metal but won't be the same as the other manufacturers, they will find sales falling as the better fakes take the market.
The manufacturers that make identical copies of the original won't have all the knowledge of the genuine product so won't know what fine internal adjustments to make to have it operating within original specifications, they will also choose cheaper parts that may not have the correct tolerance so the circuit won't perform as it should.
Take the Xterra 705 for example, the copy will still detect metal, have all the menu's, settings etc but it won't perform as well as a genuine one.
When you set the threshold the copy won't be producing the same output as the original, when setting discrimination patterns the copy might discriminate out more than just what you set it to or less.
When you swap coils the copy might not use the correct frequency for that coil or it may not recognise that a coil has been connected.