Germans strictly never drop the "und" from 21. If you swear you heard someone say einzwanzig, your ears aren't deceiving you—but they probably weren't talking about 21.
When a German speaker says eins zwanzig (which sounds identical to einzwanzig in conversation), they are referring to the decimal 1.20. In everyday life, this shorthand pops up constantly in two specific situations: money and measurements. If you buy a pastry at a bakery, the cashier will tell you it costs eins zwanzig (1.20 €). If you ask someone how wide a dining table is, they will reply that it is eins zwanzig (1.20 meters). Occasionally, it is also used for time, as in 1:20, though zwanzig nach eins is much more common.
German builds its numbers from 21 to 99 in reverse order compared to English, linking the ones and the tens with und (and). Twenty-one is always einundzwanzig ("one-and-twenty"). Dropping the connector entirely is not a recognized grammatical variation or regional dialect.
However, acoustic illusions happen during fast, informal speech. When native speakers count quickly or speak in a casual register, the crisp syllables of ein-und-zwan-zig often compress. The d and the u can get swallowed up, leaving something closer to ein'n'zwanzig. A native ear instinctively picks up on that brief, nasal n sound acting as a bridge between the numbers. To a non-native speaker focused on parsing unfamiliar vocabulary at full speed, that split-second linking syllable is easily missed, making the word sound exactly like einzwanzig.
So if the context is a price tag or a tape measure, you heard 1.20. If someone was counting physical objects or stating their age, they definitely squeezed an und in there—they just did it too fast for you to catch.
