Over-lining and under-lining are frequently done to achieve different effects. Over-lining (ie a 6 on your 5wt rod) is frequently done to help the rod load more easily... and in fact many lines are heavier than their rating just to make them easier to cast by making timing less critical.
It is common enough the line you are looking at may be a true 5wt. Every manufacturer has at least one overweight line model. Rio Gold is an example. And it usually is the cheaper lines that are overweight.
The slower the rod, the less over-lining is likely to help.
Under-lining as you are proposing is going to make the rod harder to load requiring greater timing precision (read skill) to achieve maximum distance. It is usually done intentionally in situations where the casts will be shorter and delicate presentation a must.
Just as overweight lines tend to be the cheaper models in a brand, cheaper rods tend to be slower than claimed, thus making under-lining a reasonable idea.
What are the brands and models of rod and line you are looking at?
