Corrupt, unreadable sectors on an SD card
Your confusion comes from the fact that SD cards are more complicated than they seem. Unlike magnetic disks which have physical sectors (so that sector 0 is always on the same place on the disk), flash storage is made of pages and blocks, and sectors are dynamically mapped to pages containing relevant data:


Since only complete blocks can be erased at once, writing to the SD card results in the old page being marked as invalid, while the new data for the sector is written to a new page, potentially in a different block.

Moreover, when a block accumulates enough invalid pages, it is recycled: remaining valid pages are written elsewhere and the whole block is erased. That's what could have happened when you formatted the card: your bad sectors got relocated to different pages, which are not bad anymore. If the card is smart enough, it should remember blocks which had read errors, and avoid reusing them in the future. So it's entirely possible that your SD card will be error-free from now on, until the day more blocks become unstable due to wear, resulting in more read errors.