It is only going to be as accurate as your GPS on the phone will be through iCloud. Generally speaking it is pretty good but iCloud doesn't poll your phone constantly and has some margin of error which increases as you move between polling.
iCloud polls your phone. The app polls iCloud. There can be gaps caused by the intervals. You can go look at your iCloud account and see how accurate it is.
This is incorrect.
GPS location, as a consumer service, is regulated by the Federal Government to only be accurate to within about 10'. Phone manufacturers have increased the accuracy to narrower distances, partly by adding WiFi signaling to assist with triangulation and such.
iCloud has NOTHING to do with calculation of the phone's location, and is ONLY used to store location data that the phone sends to iCloud for the Find My iPhone service on the device itself. iCloud can be completely disabled on the device and it will function correctly.
Geofencing triggers events based on transitions, not based on mere presence. At .17 miles distance from the center of a .15 mile fence, is a little over a 100' difference in distance. That's PLENTY of difference to cause the geofence to trigger a change. Do you have Location Services allowed for the Vera app?
I have everything set up correctly on my phone and have found that I need to go a few hundred feet beyond the border of my fence to get my scene to fire that handles "Leaving area" conditions while entering the area seems to only require coming over the line by 50' or so. I haven't really narrowed this down because A) the transitions always happen while I'm driving and not moving at a slow pace like walking and B) cell coverage near my house is very weak and may play a part in the delays.