The number of collections returned by db.stats() seems to correspond neither with the number of collection nor the number of namespaces. Paste this into the shell:

use teststats
db.stats()['collections'];
db.system.namespaces.find().length();

db.foo.save(

);
db.stats()['collections'];
db.system.namespaces.find().length();

db.bar.save(

)
db.stats()['collections'];
db.system.namespaces.find().length();