Why one tube has several names
Vacuum tubes were standardized separately in the United States and Europe, so many common types carry two names. The American system (for example 12AX7) starts with a number tied to heater voltage. The European Mullard–Philips system (for example ECC83) uses letters to describe heater and electrode type.
Same type vs. variant
On TubeFinder, a same type name refers to the identical tube type under another naming system. A variant is a related tube — often a ruggedized, low-noise or long-life version — whose ratings may differ. Variants should be checked against your amplifier's requirements before you swap.