In my experience this is actually true. From my plan I can see that they have calculated the payments using contractual payments as the basis. For the purpose of my plan they have set the contractual payment for each credit card/store card as 3% of the total (in reality they range from 2-4% accross the accounts I have) whilst for my overdraft they have set the contactual repayment as 5% of the total and calculated the pro rata payments accordingly, meaning a slight advantage to my bank (HSBC).
This doesn't make a huge difference to the amounts but could be where the confusion sets in
Anybody have any idea why they should treat overdrafts differently?