There is no relationship between signal strength and wavelength. It’s really simple to represent it:
when two persons are talking, one of them might have a higher pitch. But any of them can talk loud or soft. The high pitch guy can talk louder than the low pitch guy, for example.
It’s exactly the same for GSM : two emitters can send their frequencies at high power or low power. The frequency itself doesn’t have any impact on the emitted power.
Now, when you talk about interference, we can keep the same analogy:
(1) if the high pitch is much higher than the low pitch (imagine one woman and one man. When both of them are talking at the same time, you could “isolate” each speech because they are not within the same range of frequencies. It’s easy to distinguish the words from the woman and from the man.
(2) If both persons have similar pitch, it’s nearly impossible to understand what they’re saying.
So two frequencies will interfere only if they are “near” to each other… that’s situation (2).
Now, still in example (2), we could imagine that one person is talking at normal volume. The other one is speaking very very softly. In this case, you’ll have no problem understanding the first person.
If the second person talks louder, you’ll have more dificulties… and if he speaks even louder, then it’s impossible : the first person can’t be heard at all.
So you see, even if two frequencies are adjacent (nearby), the different in signal strength between each frequency is important as well.
C/A > -9dB means that the 2nd frequency shall not be 9dB stronger than the 1st frequency. If it is 8dB louder, no problem, you can understand. If it is 10dB, you can’t understand : the f2 is too strong, it hides the f1.
f1 and f2 are adjacent in your question (C/A = carrier over adjacent channel).
You could have the same example with identical frequencies: f1 = f2. In this case, C/I > 9dB, meaning that f2 shall be lower than f1, by at least 9dB. In other words, f1 should be 9dB stronger than f2. If it’s only 8dB stronger, that’s not enough : f2 is too noisy in the background and you can’t understand f1.
I hope that was clear ! With a good C/I…