The 2007 Grand National (known as the John Smith's Grand National for sponsorship reasons) was the 160th official annual running of the world-famous Grand National steeplechase which took place at Aintree Racecourse near Liverpool, England, on 14 April 2007 and attracted the maximum permitted field of forty competitors for a total prize money of £700,000 including £399,140 to the winner.
Only forty competitors could participate in the 160th official annual Grand National steeplechase and every slot was filled.
The context states that the maximum permitted field of competitors was 40 and this number of competitors was attracted to the 2007 Grand National which was the 160th official annual Grand National steeplechase. This means that every possible slot was filled by a competitor, of which there could only be forty. I think the system may have had trouble because of me using the phrase "only" in my statement - though this restriction is mentioned in the context it's stated with the label of "maximum permitted" number of competitors. Instead phrasing it as "only forty competitors" may have changed the connotation of the phrasing just enough to still be completely true but trip up the system's way of interpreting the original context.