This is due to a rather screwy yet likely simplified system of managing resources. At the beginning of each day, the game first adds all of the resources you naturally produce, and only then factors in the amount you are trading away or expending. For example:
99856 steel to begin with (whatever you seem to be maxxed out at)
+ 213 steel produced in your controlled provinces with direct land ties to your capital
+ 0 steel convoyed in (you know you don't need any more)
= 100069 steel. This exceeds the maximum allowed amount, so it stops at 99999.
- 0 traded away (you're at war)
- 143 steel consumed by your industry
= 99856 steel remaining after all of the calculations are complete.
I'm sure it would be possible to program the game to allow you to temporarily exceed 99999 long enough for you to factor in losses, but it's probably easier this way.
The reason why oil is the only one that maxxes out at 99999 for you is that it is the only resource not consumed by your industry. Only units consume oil (conversion is dealt with in the process above, but you obviously didn't need to worry about it with so much rubber), but they do not do so between days; instead, they go by an hourly system.
Actually, this is all a big guesstimation on my part; something I came up with when I was asking myself the same question a while ago. It makes sense to me, but you can take it with a grain of salt.
