Ultimately rnzoli, I believe it to be a combination of supply and demand, apparently weak or underperforming trainees 'getting better' or rather improving their skill set with continuous and concentrated repetition, and the fact that there is (as I was told early on by a senior colleague) a certain degree of protection in a multi-crew environment. To give you an impression of one or two of my experiences, I was told by a candidate on a single-engine test when informed during the debrief that they hadn't made the PFL field they selected, or the one beyond it or the one beyond that and had not been successful with the VFR Navigation section of the test: "I'm training to be an airline pilot - you don't do engine failures into fields in airliner's and they've got GPS for navigation." The last comment highlighted a fundamental lack of understanding of basic navigation principals in modern commercial aircraft, perhaps all the more surprising in view of the fourteen ATPL Theoretical Knowledge Examination passes this candidate held. Regardless, a satisfactory standard was eventually reached and this lucky young spark now flies CAT category aircraft in EU airspace - and beyond.
And I could go on, but in my experience, it wasn't always the case. There have always been weak students but I have encountered far more in the last 15 years and most with a sense of entitlement that I find increasingly discouraging. You mentioned two operators in your post; a former colleague told me recently that they have had requests for recent or almost completed students to be proposed for interview with an operator. In general, graduates who had progressed well through training, had an amenable personality and positive attitude, were rejected. Those that had not performed so well, and had not made much of an impression on the staff were accepted. You can make of that what you will I suppose.