This is perhaps the single astrological association that was always the most difficult for me (aside from the Sun in the Ten of Swords, with which I'm slowly coming to terms). As Closrapexa said, the attributions are based on a system, not on assigning the signs to the cards that fit them best. Knowing that system, and seeing how the correlation between the Chariot and Cancer was derived, I can accept it, but I agree with you that the two aren't actually terribly similar in terms of the traits they embody.
Because of this, I tend to think of Cancer as a different, hidden aspect of the personality that the Chariot can represent. If I pull Key VII in a reading and it doesn't make sense to interpret it as traditional Chariotesque restlessness, then I'll look at the card through the astrological lens and interpret the Cancerian aspects instead. Similarly, if I pull other cards that are astrologically related to Cancer (i.e. the 2, 3, 4, and Queen of Cups), then I'll remark that there's a pervasive Cancerian energy in the reading that extends beyond just the Chariot. Maybe the Chariot itself represents big plans and far-off places, but the overall tone of the reading could still be one of staying close to home.
In short, the Chariot as Cancer is never my primary interpretation. I look at the astrological association of the card as a second interpretation, to help nuance my reading or fill in any gaps that might be apparent.