p1db iip3

In many books and articles,the relationship between 1dB compression points and third order intercept points is IIP3 9.6dB higher than Pin1dB and OIP3 10.6dB higher than Pout1dB
but recently, I find some application notes that " As a rule of thumb,the third-order intercept point is approximately 10 dB higher than the 1 dB compression point for systems operating at high frequencies and 15 dB higher for systems operating at lower frequencies."
Can anyone tell me where is the 15dB from?

As a rule-of-thumb (typical behavior):

P1dB is about 10 dB lower than IP3 for a microwave RF amplifier
P1dB is about 15 dB lower than IP3 for VHF and HF amplifiers.

There is no exact analytical relationship between P1dB and IP3.

In fact, There is no relation between P1dB and IIP3. The relation in textbook is based on linear assumption.