Forgive me if I am wrong, but here is maybe why.....
You are tapping (punching) in a destination, by which I guess you mean a town or a city, as you know it is somewhere near where you want to go to. For instance, you know you want the A3 so you tap in Guildford, as you know that will bring you close to where you want to go.
The device knows where Guildford is, just as you do. The only problem is that it only knows Guildford centre, not that specific bit of Guildford that you had in mind to take you to the specific stretch of the A3 you wanted..... So it takes you to exactly where you asked for.... Guidford centre, with boring reliability.
Of course you'd be mighty pissed off if you wanted to go to say the middle of Coventry and it kept dumping you off on the ring road. So it cuts both ways.
Yes, you can avoid going into the town centres when out and about and without the luxury of your PC to hand, in several ways. Here are two of the easiest.....
1. Look at the route it suggests you take. Zoom the map in / scroll it around (easy on a 660) and tap on a road that you know will suit you. The dumb device will then ask you if you want to set that point as a via point or as a new destination. Chose, new destination. Bingo, job done. Or you could selct via point abd the chances are the route will shift to excatly where YOU want it to go. Repeat as necessary. You can edit routes on the device, just as you can on a PC at home, it just takes a bit more effort. For instance, you can re-order via points... Play around with the thing, it really can't break it.
2. Ignore the bloody instructions the dumb device is offering when it routes you into the town centre. Best done by the old fashioned method of engaging brain. This works for many other routing problems, too. So feel free to refer back to it as often as you like. For instance, if you are hammering along the Guildford bypass (which you are happy with) but the device wants to send you to the middle (for reasons known or unknown to you) ignore its bleating.
PS One last tip. Turn the bloody ping prompts off and the damned voice off too. You don't need to be told where to go and it makes you lazy. When you get lazy you make more mistakes. Look at the screen (it's quite safe) and think where the route is taking you.... This is much easier without the voice, as it forces you to make more conscious decisions of your own. Yup, you've spent hundreds on the Bluetooth and Autocom... Junk it. You will get used to it very quickly, trust me.
Oh, and use the very nice zoom in-out features as you hoon along too and play with the map orientation. Yup, it means tapping a screen (just like naughty car drivers texting, but we are on a bike so that's different, mate) or whirl the nice iWheel on your super 1200WC. This (without the voice) will get you out of more scrapes than you can imagine. In short, be brighter than the device.... Though the device is really very clever.
Cheers for the tips.

When I was in Italy, we stayed on Lake Varese, and I took myself for a bimble one afternoon, as Paul was planning the next days route. I fancied going to Lake Como, so put in one of the small villages on the lake itself, rather than Como. The nav then routed me through the middle of Varese, round various one way streets, and then through several other towns. As I didn't know the area, and hasn't been there before, I had no idea where I should be headed so just followed the nav.
I only had a couple of hours spare, so didn't have time to plan the route properly, but it was annoying that the nav insisted on sending me through all these towns, with all their traffic, when avoid traffic, and fastest route are set as default.

Coming from a TomTom, it'll take some getting used to, and I hadn't used it that much before the trip, so will persevere with it.

