Black point issue

I have an issue with dark tones printed on Mat Archival Epson paper with an Epson 2400, MacOS driven. I print from Photoshop, through a personal ICC profile that I obtained with an Eye-One kit. I print a grey chart with patches at L=0,5,10,15 etc. in RVB mode, then I measure L values on the paper with the Eye-One spectrometer.

According to ColorThink, the black point of my profile is at L=16 and this actually corresponds to the black patch in the calibration charts used to derive the profile.

My point is that when I print with relative colorimetry rendering +black point compensation option, the “black” printed on the paper is around L=19-20, noticeably less dense the expected L=16 value. What happens ?

I add that
(i) passing from L=16 to L=20 is rather sensitive on matte papers, unfortunately
(ii) I recover L=16 when printing with perceptive rendering… but then the darkest printed tones are not regularly spaced in density.