I had to use radComboxtBox and aspDD on the same page once. I observed that, for the same value of width they had different sizes. I it was pain to find each telerik combobox and make it 305 or sometimes even 308 or 310, to look same as asp:DD having 300 width.
I think this type of issue raised in DropDownList/Calender or calender/RadCalender.
Because they are used different type of contolrs/tags to generate it.