Introduced as villains in Dragon Ball Z and now full-fledged heroes in Dragon Ball Super, Androids 17 and 18 have left an indelible mark on the long-running series. Dragon Ball's fanbase has long since suspended their disbelief for the franchise, so when an evil scientist created two killer androids that looked like teenagers and were more powerful than Super Saiyans, no one batted an eye. However, in accepting the vague explanation of how Androids 17 and 18 came to be, many people may not realize that the androids aren’t actually androids at all.
The formal definition of an android is a robot that has been made to look and act like a human being. However, Dragon Ball Z has established that 17 and 18 are not robots, confirming that they were born as ordinary humans named Lapis and Lazuli. It was only after they were kidnapped and experimented on by Dr. Gero that they became known as Androids 17 and 18. It's hard to think of them as anything but Androids 17 and 18 now, but in the end, their classification is factually incorrect.
A living organism that has been enhanced with robotic or mechanical parts is termed a cyborg. This definition falls much more in line with what Dr. Gero did to Lapis and Lazuli, and somewhere out in the vast multiverse, there must be a version of this series where they have always been known as Cyborgs 17 and 18. Starting out as humans, whatever Dr. Gero did to them has become so ingrained into their bodies that Shenron himself cannot undo it. This would also explain why Android 17 was resurrected by the dragon balls when Android 16 was not -- because 16 is actually an android.
In fact, Androids 17 and 18 appear to be the only of Dr. Gero's creations who are not androids. Non-canon Android 13 and his trucker hat aside, Androids 16 and 19 fit the definition of an android. When 16 and 19 were blown to smithereens by Cell and Vegeta respectively, they spewed machinery everywhere, confirming that they are indeed robots. Fortunately for 17 and 18, they have never been blown apart to find out what their insides look like, but that they are still mostly human is proven by the fact that by Dragon Ball Super, both have produced children.
It's hard to imagine that Dr. Gero could be brilliant enough to create androids and cyborgs, but not brilliant enough to know that there's a difference. For an evil genius, Dr. Gero is a pretty simple guy. He creates something, names it Android insert-number-here and moves on. Perhaps he just didn't want the headache of keeping records and drafting schematics for various androids and cyborgs with possibly overlapping numerical designations. However, considering that he kidnapped Lapis and Lazuli, tortured them, turned them into cyborgs and was then surprised when they turned on him, maybe he's not that smart after all.
At the end of the day, there probably isn't an in-world explanation for the misclassification of Androids 17 and 18. Akira Toriyama, the franchise's creator, must have thought android sounded cooler, or perhaps he didn't know there was a difference either. Nitpicking aside, it makes no difference to the series that 17 and 18 are termed incorrectly. Still, as was said best by Imperfect Cell in Team Four Star's famous parody Dragon Ball Z Abridged, if you call 17 and 18 androids, "good for you, but you're wrong."