Wow, well this will take some explaining... I thought you would know this already SE, being a Supes fan. But for the quick answer, that is how post-Crisis Supes' powers work. Lois and Clark series came out after Crisis, and so it follows that version of Supes (in other words, the show borrowed from the comic, not the other way around).
Okay, the very first Superman would be called Kal-L, this is the Golden Age Supes. Originally he could leap about an 1/8 of a mile, but slowly became more powerful and could fly, works for the Daily Star, etc.
Next you would have Kal-EL, this is Silver Age Superman. He has a significant increase to his power levels, works for the Daily Planet, revealed himself to the world as a teen (Superboy), had met Lex in Smallville before they became enemies, etc. Here is also introduced the idea of the Multiverse. Kal-L from Golden Age is from Earth-Two, while Kal-El from Silver Age is from Earth-One.
After this you have the miniseries Crisis on Infinite Earths (this is where you get pre-Crisis, post-Crisis). This would be in the mid-80s. LONG story short, it explains the origins of the Multiverse and eventually a new singular universe is made that has combined aspects of the different parallel worlds.
Post-Crisis, you get the Modern Age Superman introduced in the Man of Steel series. This is where Supes powers are described as I say, with his force-field and such. In the Silver-Age, his outfit was invulnerable b/c it was made from cloth from Krypton.
After this there is also Zero-Hour, which changes stuff up yet again (but this time it deals with time travel instead of parallel worlds).
And very recently you have Infinite Crisis. Lex jr. and Superboy-Prime have made a dimensional tuning fork from the remains of Anti-Monitor, and tricked Spectre into destroying all magic to create a power source. Then with the fork they begin recreating the alternate earths.