AIBU to not understand why the oven takes longer than the instructions state?

[BerryCheesecake]

Is it just me?!

Every oven I've ever had doesn't seen to run to time correctly.

I heat the oven up beforehand to the required temperature, and put the food in. After the required time on the packet, I check the food. It's never cooked
I usually give things about another 10mins and the they're cooked. Am I the only one this happens to?

If not, what am I doing wrong?!

I've often noticed that too OP. I sometimes wonder if the instructions are for the absolute minimum times (assuming oven up to temp and cooking evenly throughout) to encourage as many people as possible to buy the product (longer cooking times being a disincentive).

I have to say my almost new gas oven is a pile of shite compared to my knackered old fan assisted electric oven
You can only cook on the top shelf, the middle shelf can only be used to keep food warm
Op do you need to pop your shelves up a notch?

I agree with all the comments about clean ovens working better, an oven thermometer helping, checking your door seal - I got a new oven recently & it works so much better than the old one for these reasons. I also realised that keeping my baking trays and roasting tins in the oven mucks it up if you don't take them out before using it!

Surely whatever it says on the packaging is a guide? I never pay attention to it and just use my common sense!! Just don't understand those people (like my dsis and bil) who serve undercooked food on the basis that it must be right because the packet said so...
Our old oven was rubbish and home made pizza took 20 minutes and had to be rotated once because otherwise only one side was cooked properly. Our new oven cooks the same pizza in 13 minutes (without the fan on!) and needs to be watched. Just use your brain and you are fine....