Heart Rate Zones

According to this, I shouldn't be in Zone 5 for too long and yet I hit 50% in the workout I just did. I didn't feel like I was overdoing it. Sure, there were times that I needed to take the foot off the pedal but I listened to my body and did so. Should I ease off or is this just a sign of my fitness improvements? I've been exercising 4-5 times a week (I've just started my 5th programme with FB) since May and I feel so much better for it (apart from my smashing my hand into a door handle during a lateral jump and ripping a chunk out of it - could have done without that).