I have never seen a Tiger get killed in a single hit from the front by a T34/76.
I've seen a kill happen from a T34/85 from the "front" but that simply was front FACING, rather than the front ARMOR. The Tigers in these cases were actually going downhill or had the tops of their hulls exposed which, assuming Tripwire modeled top armor as independent of other armor plates, would make sense as a penetration and potential instakill.
The ONLY situation where I could see a T34/76 doing this is if it's actually hitting the top armor.
Now, given that some people DON'T use the range adjusters and instead lob their shells at the enemy, I suppose it IS possible that Tigers are getting penetrated when facing towards the enemy BUT hit from the top and the angle of attack lines up with the ammo store.
But a true head-on shot will, at most ranges, deflect. I've tested at short ranges (<200m) and a T34/76 CAN penetrate the Tiger's frontal armor enough to make it "go yellow", which presumably means it can penetrate enough to score an instant kill if you aim right. But the stories about "I was instakilled in my tiger by a Mosin Nagant 91/30 at 20000m" are, I think, slight exagerrations.
Like I've said, people tend to overestimate range in the game while in tanks. Go and practice on Mormegil's tank range map (do a search for posts by him or for "tank range") and you'll start to see that your "OMG!! IT WAS A 1500m AND I GOT KILLED IN ONE SHOT" stories are probably closer to 700m, if that.
Overall, I think we've covered what the issue is -- short range penetration is slightly overmodeled, as is the effect of angling. Some vehicles are indeed buggy (SU-76's upper frontal armor, for example). But long range penetration is GENERALLY accurate, if you play on long range maps. Arad is NOT a long range map, by the way. It's a mid-range map. Even the later versions of Black Day in July are mid-to-long but not extreme long range.
Seriously, as slow and boring as it may be, your best test of penetration power is Orel76v88. If you REALLY want to match up Tigers and T34/76s and see who comes out on top, play that map. One on one head-on engagements are almost guaranteed to go to the Tiger.