As an average wall or the body of a car can stop alpha and beta radiation, only the gamma radiation is a real issue, but that part could be much lower than the measured value.
Unfortunately, it's not quite correct because isotopes can be consumed with water or food. Unlike gamma, alpha particles are able to give all of its energy to cells. That's why alpha radiation from Am241, Pu238-242 and some other isotopes is the real threat as well as reason of evacuation from the Chernobyl and Fukushima exclusion zones. On the other hand, people are able to live in gamma contaminated areas without noticeable problems. Just for example, 25% of Belarusian territory is contaminated with Cesium-137. Due to weak interaction with matter, gamma radiation is not a real issue unless there are really high levels of activity.
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It should be noted that most of the dosimeters can see only gamma and hard beta radiation, this makes them useless for the assessment of a real threat.
If you need an explanation then there is an example:
Americium-241 emits alpha particles and weak gamma rays;
Cesium-137 emits hard gamma and beta rays;
If you would use typical geiger counter to compare Am241 and Cs137 sources with identical activity, then you will see lower CPM values for Am241 source. This will inevitably lead you to wrong conclusion.
