Sidebar:
London Broil is not a cut of beef, it's a cooking method. Typically, flank was used. When flank became a popular menu item in the early-mid 80s (served grilled, sliced and fanned on the plate; called 'flank' on the menu), demand went up--and so did its price. In not a few areas it was difficult to come by on the retail level.
Rather quickly, to counter the resulting dearth of flanks for London Broil or consumer complaints about the significantly higher flank prices (the name and associated technique had been around for a while--probably since the 30s--and consumers were not happy), retailers started packaging other cuts as 'London Broil'. Sirloin (usually top) and, mostly, top round were labeled as such (with or without the actual name of the cut on the package). Because it was so often without, many people came to think of LB as a cut instead of a technique.
Though I've heard of this before, I'd not actually seen it till last week while scanning the meat case at a local store: There in the case, two packages, each labeled 'Chuck Shoulder for London Broil'. Couldn't tell from where it was cut.