When I say 30 seconds, I mean 5 seconds here, 5 seconds there for a total of ~30 seconds over the duration of the interval. In theory that will drag your average power for the interval down 5%. When you throw in shifting, inclines/declines, tight corners, passing people/riders (where power inevitably drops momentarily) that means I'm surging up to 110-115% to drag average lap power up. Even on climbs I find it hard to sit right at target power and have it closely track average lap power. When the grade flattens out for a bit, or I clear my six to go around a parked car in the bike lane, power can drop 100+ watts in a second or two. It means I'm pretty much always doing at least 5% above target power when pedaling. And even then I usually fall at least 5-10w short of my target average lap power.

Part of me thinks that riding at threshold for 19'30" gives you at least 95% of the benefit of the full 20'. If you go by the book it seems like you have "failed" the interval.