Denny Vrandečić, 07/03/2013 19:02:

the number of active editors that I get here

<http://stats.wikimedia.org/EN/TablesWikipediaSIMPLE.htm>

is very well defined, and says what it means.

Yes.
Remember that those are defined on the calendar month, while "Contributor" is yet another thing and measures from beginning of time: https://www.mediawiki.org/wiki/Analytics/Metric_definitions#Contributor

But trouble is, it is
inconsistent with the number that I get here

Why is it a trouble?

<http://simple.wikipedia.org/wiki/Special:Statistics>

for active editors, and if I understand the definition at the latter
correctly it means "one edit or action within the last 30 days".

Yes.

Can someone please help me to interpret the numbers appropriately?

It's not about interpretation but about purposes. :)
1) To find someone who can help you on a wiki, use the query for active users: it's better than skimming RC as we used to have to.
2) To have an idea of the top-activity or core community health, use the "very active editor" of any given month.
3) To have an idea of the total activity on a wiki, use the number of active editors.
4) To know how many new editors a wiki is attracting (not caring of how many are being lost), calculate differences between numbers of "Contributors".

Nemo