Ooh! It's just like a class assignment! Sun letters are alveolars. It looks like that expands as far as interdentals (th) and alveo-palatals (sh). By the way, what's the difference between s and S?
I'd bet that the assimilation of the lateral in the article to the alveolar is standard anticipatory assimilation of homorganic phonemes. Since the l also touches the tongue to the alveolar ridge, it's basically skipped since the next sound is also an alveolar one.
One of the most common examples of anticipatory assimilation in English is nasals. Some people say income as in-come, but most people (especially if you're de-stressing it, as in the compound income tax) will say ing-come. That's because the next sound is a velar stop, so they make the nasal a velar nasal. It's essentially "lazier" than doing an alveolar nasal and then transferring the tongue to a velar stop.
Similarly, you have intolerable, interminable, indifferent, but also impatient, impotent, imbalance. You could say that there are two prefixes that mean "not", and they are in- and im-, but what makes more sense is to say there's an iN- where the nasal assimilates to the next articulator.

OK. I should go do some work now. Thanks for letting me spew linguistics all over you.