I found this answer helpful:
1000BASE-LX is specified to work over a distance of up to 5 km. In practice it will often operate correctly over
a much greater distance. Many manufacturers will guarantee operation up
to 10 or 20 km, provided that their equipment is used at both ends of
the link. 1000BASE-LX can also run over multi-mode fiber with a maximum
segment length of 550 m.
I think the right answer is that the 802.3z standard specifies 5km as the max, but many manufacturers (incl. Cisco) specify 10km as the max b/c of high quality equipment.
I passed the CCENT 2 months ago and there were no ambiguous questions that provided both lengths.