Why does this happen? [having to scan a bar code more than once]

Looking at your screenshot, it did not scan the full barcode the first time, it missed the EXTension.
TIP: set the scan screen to EXT mode to force it to look for the barcode extension.

Note: don’t forget to switch it back to Auto when scanning barcodes without extensions, e.g. older comics or TPBs/HCs.

Thanks for the tip Alain. I just “assumed” the barcode didn’t scan properly but now I know to ensure the ext part is scanned too.