Because it implies that the original plan was to continue the ongoing numbering from the final issue of the STAR WARS MONTHLY which, despite four title changes (STAR WARS WEEKLY, THE EMPIRE STRIKES BACK WEEKLY, THE EMPIRE STRIKES BACK MONTHLY and STAR WARS MONTHLY) and a frequency change (from weekly to monthly) dated back to the launch of the British comic in February 1978.
ROTJ eventually appeared with a brand new first issue… and clocked-up 155 more issues before succumbing to saga apathy (and Marvel NY abandoning the license) three years later.
It's an odd selection of art (from the official adaptation) as well. Darth's definitely the iconic image but waggling his finger at a lackey is hardly the most dramatic moment in the movie. Maybe all the good stuff was embargoed!