Is 'Unemployment is at a 10-year low' standard? Why not “the lowest level since 10 years”?

Question

Unemployment is at a 10-year low, wages are rising at a robust rate, and the state budget ran a surplus last year, yet ČSSD approved a long-term agenda over the weekend that states that the low overall tax burden is undermining the state budget and forcing broad cuts in spending.

Source: http://www.fsfinalword.com/data/FW170313.pdf

1. Do you think that the passage in bold is standard? "Low" is after all the adjective so why is there the indefinite article?

2. Is it possible to rewrite this sentence this way: Unemployment is at the lowest level since 10 years?

Answer 1

Unemployment is at a 10-year low

Low acting as a noun Dictionary.com here, is not a definite "low", but represents the lowest level for 10 years (there might be levels as low in that period). The use of the indefinite article suggests that this "low" is not unique (unique would be "the low")
This is acceptable usage.

Unemployment is at the lowest level since 10 years

The reader will wonder what is meant. Standard usage would be:

Unemployment is at the lowest level in 10 years

However, since could be clarified:

Unemployment is at the lowest level since 10 years ago

and the reader will immediately understand which ten years is involved. "Since ten years" is not specific enough for a lot of readers to immediately understand.