Well my three
tac squads are almost finished (pics soon) and I don't know if anyone remembers, but approximately 1/3rd of my Marines are "reclaimed" models that I butchered as a kid. I covered up the glue damage and paint that I wasn't able to strip with hooded tabard-based robes... More like Black Templar ones than Dark Angels', pretty much because a tabard is easier to make

(I failed at doing
DA robes).
I posted pics before but here's one, I'll get others up as soon as I can coax my sister to lend me her camera again:
At any rate, here's my fluff excuse for such a discrepancy:
Elements of the Dark Angels 5th Compan, led by the Company Master, Artemis, were despatched to deal with Chaos Renegades whilst the remainder of the Company remained with friendly Imperial Forces suppressing dangerous cultist activity on a major world. The enemy, like the cowards they are, fled and the Dark Angels pursued them, distancing themselves further and further away from friendly support. It was only once the loyalists were dangerously exposed that the enemy suddenly broke off their retreat and their leader revealed himself: A fallen Dark Angel. Artemis formed a Strike Team from his veteran Devastators and Command Squad. However, the situation was grave and it was possible that the Space Marines faced annihilation and with no time remaining in which the Deathwing could be summoned, it proved necessary to commit more resources to a boarding action against the Renegades. Thus, with the Company Chaplain's approval, Artemis elevated his most senior Tactical Squad (#1) to Company Veteran status... With such little time and without the Chapter Artisans, the squad quickly set to work creating, to the best of each individual's ability, their own bleached robes, a humble imitation of those worn to express penitence for the fall, but far less elaborate due to the constraints of their mediocre abilities and the lack of time available.
Against all odds, the action was successful, the Fallen captured and the enemy routed... 1st Squad continues to wear their self-crafted robes as an expression of pride for this action and are slowly being filtered through into Artemis' retinue and from there, on to the Deathwing, as casualties allow. (An excuse for some of his retinue wearing these tabards and for me mixing proper robes in with my green stuff ones).
Any criticism is welcome, this is just a story I would tell if anyone said to me "those Dark Angels aren't wearing proper robes": I only take this precaution because I myself am a bit of a fluff-nazi and would probably ask the same thing.
It doesn't seem unreasonable for this to happen under exceptional circumstances, since in Angels of Darkness: