PIFs are correctable as long as you got fewer than 4 of them per ECC block. A block with 5 PIFs or more produces a POF.
Actually, the reading drive itself decides what is a POF and what is not. POF simply means that the drive has failed to correct. Some drives can correct extremely high PIF levels, and others can barely correct any. (on a given disc or media type). The reading speed is also a critical factor. Therein lies the trouble with error measurements, it's all relative.
The specs, (280PI/4PIF) are just standards for the maximum "allowable" errors. Each drive decides for itself what the error levels are and how to deal with them, and when to fail. DVD players are notoriously inept at error correction, and do not really adhere to any known "standards". Where the error happens to fall in the datastream is also a factor.
To summarize in a simple way, there are several levels of error correction. The first level is called PI. IF an error is not corrected at the first level, it becomes PIF and a new (second) correction scheme is applied. If that fails, you have an unreadable sector referred to as POF.
Also note that read speeds plays a huge role. In scanning tests, a drive is locked at a set speed. In real life, the drive can slow and re-read a block to correct an error where in a scan it cannot. So, a failure in a scan does not always represent a truly "unreadable" block. Confused yet?