Yeah, that confused me the first time too. 一人(ひとり) is just a weird pronunciation if I remember correctly.
七人(しちにん) actually makes sense because both are on’yomi readings.

人 is the kanji for person. It has two on’yomi readings にん and じん. It’s kun’yomi reading is ひと.
七 is the kanji for seven. It’s on’yomi reading is しち. It’s kun’yomi reading is なな.

When it comes to numbers, they usually have one on’yomi reading when your just counting in general. eg. 1, 2, 3 ,4, 5 etc. and a kun’yomi reading which is used for stuff called counters. However, the numbers four and seven are slightly different.

Four has two on’yomi readings し and よん plus a kun’yomi reading used for counters. When counting in general you can use either し and よん but for certain numbers like 40, 400, 4000, only よん is used. For counters, both is used. You will have to learn when which one is used unfortunately.

Seven has one on’yomi reading しち and a kun’yomi reading なな used for counters. なな can also be used to count in general. Like with number four, なな is only used on certain numbers like 70, 700, 7000. In some cases しち is also used as a counter. Like with four, you will have to learn when which one is used.

The numbers nine and ten have two on’yomi readings but only one reading is used for general counting and the other on’yomi reading is used as a counter. They also have a kun’yomi reading used for counters aswell.

As for your question, I recommend reading this article on Japanese counters http://www.tofugu.com/guides/japanese-counters-guide/. Counters can get very confusing. In some cases it’s easy to work out the reading of a counter, other times you have to learn the exceptions.

As for your question. The general counter for counting people is にん. e.g. So to say three people it’s さんにん, five people ごにん, six people ろくにん.

七人　じゃありません means it is not seven people. This is a sentence about counting people. On this occasion it doesn’t use なな, it uses しち.

一人　(ひとり) is the counter for one person. It can also mean alone. It doesn’t use いち or にん. So saying いちにん would be incorrect. Fortunately the counters for one person and two people are the only exceptions.

一人　でした can mean two things. It was one person, or (I) was alone.

Apologies for such a long reply but It was easier to explain it in full. If you have any more questions, don’t hesitate to ask.

• This reply was modified 12 years, 5 months ago by  kanjiman8.

Thank you very much for the answers. I just came across this practice lesson today and thought the sound was different too. I’m following the lessons in the right order since season 1 and didn’t find anything regarding this exception =/