Similarities between dissimilar sounds

It may be just me, but I hear a strange similarity between sounds that are clearly different, viz.

/3i/ as in sexto ^©,
/3~i~/ as in quem ^©,
/o~i~/ as in plural words that end in -ões. Examples: eleições (elections) ^© and condições (conditions) ^©.

on the one hand, and the Dutch sound written ui (phonetically [9y]) on the other hand. An attempt to explain this phenomenon:

All four diphthongs go from half-open in the direction of a close sound. That is, /o~i/ could be expected to start half-close, because the monophthong /o~/ is also half-close, but in fact it usually starts much opener than that.
The Dutch diphthong is front (but not quite) and rounded, the Portuguese diphthongs /3i/ and /3~i/ start central and unrounded. The acoustic effect of rounding a front sound, and centralising a front unrounded sound is different, but often seems similar, to my ears at least.

(Also read and listen to this).