The confidence level is between 0-1, so a value of 0.08 means Google has a confidence level of 8%.
I played with it and found some interesting results. First, I tried the word bell. Google thought it was English with a confidence level of 4.75%. Then Jingle bells. Surprisingly, two English words got a lower confidence level 1.36%. Well, that may be because jingle was an English word with a confidence level of 0.27%, and bells (with an s, it went lower to) 1.62%. But Jingle bells got a confidence of bells minus (yes, minus, not plus) Jingle.
Let us continue --
- Jingle bells, jingle bells, (1.36%. Repetition does not increase the confidence.)
- Jingle all the way; (32.91%)
- Oh! what fun it is to ride (59.7%)
- In a one-horse open sleigh. (34.12%. The confidence drops.)
- the whole thing (Jingle bells, jingle bells, Jingle all the way; Oh! what fun it is to ride In a one-horse open sleigh.) is 81.57%.
- plural form (or other forms) lowers the confidence;
- more words may lower the confidence;
- repetition does not increase the confidence;
- your input history does not help Google to build up the confidence;
- overconfidence is bad. Google is not 100% confident with any words, so Google is conservative.