What is the origin of the phrase signal improvement?

I have checked all the websites that collect info on phrase origins and none of them had anything to say about the expression signal improvement. Sending signals by semaphore and fires like in the Great Wall of China are very old so it could have its origins in that activity. My understanding of the meaning of the expression is that a signal improvement is an improvement that is really significant and stands out from all the rest. I have tried to find a definition but, again, no luck. The expression exists back to 1800 on Google NGram.

The sense lies in the word signal: OED

• Notable, conspicuous; remarkable, striking.

• a. Of an event, action, or quality.

As in:

2000 N.Y. Times Bk. Rev. 23 Jan. This gathering of data is a signal achievement.

The OED find this early usage:

1591 G. B. A. F. tr. Discouery Subtiltie & Wisedome Italians xxxvii. 57 Now what nation hath that been..which hath done God so signall a peece of seruice. Surely it was England.

My sense is that there is no idiomatic phrase as 'signal improvement'.

Etymonline reports:

"remarkable, striking, notable" ("serving as a sign"), 1640s.