I don't know if this is the rationale for the discrepency or not....however I think it makes sense.

As you've noted many times, determing that an event qualifies as MACE by the company's narrow definition...this entails a process of adjudication that can take some time.  Just pulling random numbers out of my nether regions let's say that on Jan 1st 2019 that BETonMACE had confirmed 220 events, with another 40 events still undergoing adjudication....I don't know the process for this adjudication, or how long it takes....but obviously as adjudication is taking place the numbers are constantly changing.  

Events are moving from the suspected pile to either the confirmed pile or non-event pile...meanwhile more suspected events in need of adjudication are being added.

When the 250 confirmed event number is reached...let's say there are still another 30 events that need to be adjudicated.  Perhaps we'll get up around 270 when all is said and done....

That might be the reason why the plus sign was added on slide 20.  I'm thinking the trial would stop at 250 confirmed events, and then that number would be added to as further adjudications take place even though dosing has stopped.