Your data sets are too small for you to say that it's proof. What you have is that HTC made 2 Google phones and then Samsung made 2 google phones.
The HTC G2 was a pure google phone, that puts HTC at 3 Google phones. The original motorola Droid was a google phone, that puts them in the mix with one. In terms of pure Nexus devices, it's HTC 1, and Samsung 2. In terms of pure Google experience, HTC has at least 3, Motorola has at least 1 and Samsung has at least 2. That's also assuming I'm forgetting a device or two, which I probably am.
Therefore, there is no pattern and the next Nexus will likely remain in Samsung's corner since it's based on who bids the most and not on "who's made what". Samsung will likely outbid everyone this year and it will like be the samsung galaxy nexus s or something absurd. I'd rather have a Moto Nexus personally (hey why not, give them a swing at it) and have it not be a Verizon exclusive, but that's a pipe dream.