sometimes it is less then stellar materials used in the clones. where one may be stainless and the clone is chrome plated mystery metal.
things like how well everything is machined how good are the threads etc.
sometimes the metal itself can be thinner, not a big deal on some items but for example you buy a high end dripper then get the 5 dollar clone and see the metal is half as thick. the posts are thinner and weaker and the screws are plated.
another good example is even if it looks the same things like switches and buttons may be cheaper versions not built the same way or as well as the originals. mystery metal springs. unknown coating. and all kinds of little things.
it really depends on the actual clone you are trying to compare some are made quite well and can be an identical copy in every way,,, others not so much.
then there is the belief that many developers who seem to put out a lot of stock all the time and seem to have no problems with restocking all the time may actually be using the same factory to produce their item as well and the identical clones may in fact be seconds or a second line at less cost just to get the sale anyways.
a good example of this is a
mod sells out yet they restock like clockwork while another
mod restocks after 6-8 weeks sells out in two days then its another long wait for more.