wrong fuses in fuse box

[rossman]

I have 240z with what appears to be a completely unhacked wiring system and unmelted original fuse box. As far as I know, the rear running lights have never worked since I got the car. I've had the car for almost a year and I haven't noticed it since I almost always drive during the day. I found out the rear running lights were out when I got pulled over by the local police. Luckily he let me go with only a warning. Today I decided to fix the problem. The first thing I did was check the fuse. And as luck would have it, the fuse is blown. Upon inspection I found a 10A fuse where the lable on the cover specifies a 20A. I think to myself: Well the PO must have not had the correct fuse so he just installed a smaller one to get by until until the next trip to the auto parts store. So I replaced the fuse, no big deal. Then I start checking the other fuses and find that several of them are either over or under sized: the right head light - 5A, Horn - 20A, Air con - 30A, and another lower one, I don't remember which. So, other than not having the correct fuse at the time, why would somebody put different rating fuses in the fuse box? I'm a little paranoid. I don't want to burn up my car