I think it's because (applies to all plurals) Dutch don't pronounce the final n from en. The spelling of plurals on Duolingo is more popular in the Flemish Region.
On a different note, I don't know where to report this, but I'll give it a try here hoping someone will see this. You can strengthen the "Questions" lessons by completing this lesson of "Numbers". I think that happens because of this question.
That is very interesting! I am using Duolingo to learn Dutch while living in Antwerp with two girls who speak "Flemish" Dutch, and they are always laughing at how "Dutch" the pronunciations are. Their dialect is really quite different, I think. So they correct some things so I speak more 'Belgian'. But at least I know I'm doing plurals right!
I think it depends also on where in the netherlands itself you are. I asked my dutch flatmate who is from maatricht about this and he says to pronounce the "n" s. This matter really confused me at the beginning as i was also using Rosetta Stone simulataneously to Duolingo and there they teach you not to say the "n"
The present continuous for "to have" is an exception, because it changes the meaning of the verb (from possessing to consuming). Similar exceptions are "to see" (I'm seeing him would imply that we're dating) and some others. Since those special meanings are not there in the Dutch sentence, you can't use the present continuous in your translation for these verbs.