Joel is correct the first 2 counters are exceptions to the rule.

Mostly there are 2 different counting systems in Japan one is the traditional counting system, and the second is the Chinese counting system imported along with Kanji.

So 一人 and　二人　use the traditional Japanese counting system. Where as the rest use the Chinese counting System.

The Japanese counting system is used for small items such as food or beverages in a restaurant, pub or my personal favorite festival food booths.

一つ　(ひとつ）、　二つ　（ふたつ）、　三つ　（みっつ）、　
四つ　（よっつ）,　五つ　（いつつ）　、 六つ　（むっつ）、　
七つ　（ななつ）、　八つ　（やっつ）、　九つ　（ここのつ）、　
十　（とお）。

I’m only covering the counters through ten. For further study [1].

[1]
http://en.wikipedia.org/wiki/Japanese_numerals

I know your question has already been answered but now you know why they are exceptions to the rule.
　

• This reply was modified 9 years, 8 months ago by  Devon Lazzari.
• This reply was modified 9 years, 8 months ago by  Devon Lazzari.
• This reply was modified 9 years, 8 months ago by  Devon Lazzari.