snapping cannot be off in navigation, but there is limit - snap cannot be unlimited yo may have problem with your GPS, try to reboot your device, it often helps
I agree with tomas. On my Xperia active routing is generally snapped to the route. In some rare cases, the cursor is beside the route. For me it is obvious that it is a GPS problem. That means, that gps signal is weak and/or gps location is quite far from the route. If the cursor is beside the route, I understand it as a warning for a weak GPS signal.
I also agree with Tomas. It just happened last Friday to me when I was driving in a city between (high) flats. Obviously GPS reception was partly blocked by the buildings and my position was not very accurate.
on this note....I experienced this quite a few times lately...when I wait at the lights or just in general...and I am locked to 10+ satellites the "arrow" just starts jumping around. After it changes several positions on the map..it gets back to the road when I start driving...it's really annoying.
Nexus 5 - 5.1 lollipop
Edit: also..when I stop..I think it (the arrow) should always face the direction of the current route. 99% of the time it just rotates to a different direction...so in my opinion there should not just be a lock on the road but also.... lock the direction (of the arrow) to the direction of the route
I have also mentioned the arrow problem failing to point in the direction of previous travel when stopped. This has been happening in many versions of navigator.
There is nothing to do for you: When you are navigating, the array is automatically locked to the nearest road. If you are not navigating, but simply studying the map, the arrow is displayed according to your current GPS position only.