There is a cap on electronics in the performance plan (£7 i think), but there isn't one on the classic.
However as far as i
am aware on the classic, you can't get commission for third party electronics sales.
There are also different bonus amounts for linking methods:
Qualifying Direct Link Items Premium
(Premium applied to referral rate) 2.5%
Easy Links Premium
(Premium applied to referral rate) 1%
These may factor in your commission percentage.
The referral rates are as follows (total items dispatched)
1 - 5 5.00%
6 - 20 5.25%
21 - 50 5.50%
51 - 100 6.00%
101 - 175 6.50%
176 - 500 6.75%
501 - 1600 7.00%
1601 + 7.50%
If your rate is higher than the tier then it may be to do with the bonus for direct linking, if lower it may be due to high value items being capped.