A character can be expertise with no need of many ranks with talents, we have a talent that gives us +40 OB.
Wow. That's quite the talent.
What I mean is that exists in character law, not only for us
IMHO talents reflects natural aptitude more than expertise.
That's right, but the problem is that aptitude is part of "how much good" are you in the area, so the item bonus should be used in skill bonus base instead ranks, more proper (# ranks) for maneuvers and things like those.
But fortunately in RM there is no problem as it is a 100 based system, you can use the number you want to give full bonus, and reduce it for lesser skill bonuses.
i.e. we can use 80 as full item bonus, so if we have +40 in the skill, we use only half. Using 'jumps' in bonus is recommended in +5 increments for easier management.
i.e. (more detailed), using 80 as full item bonus: we have a +15 item, 15/5 = 3 jumps, so 80/2 = 40 (if decimals then round to the nearest 5). Then each +40 we apply another +5, but starting the first +5 at any bonus, as quality items always are better that non-quality ones, for this we use 80/(jumps - 1) to compute the intervals.
In detail (skill bonus interval -> item bonus used): (-infinite)-39 -> +5; 40-79 (+10); 80+ -> +15. If it is easier add +1 to intervals, so the full item bonus is with 81+ and +10 in 41-80.