Wrong process result when use osmosis to "FILTERING DATA"

[Dancingmeow]

I'm trying the example from LearnOSM. However it seems an incorrect result is got.

Filter all schools from sample_osmosis.osm.pbf

rem use osmosis to get result
osmosis --rbf sample_osmosis.osm.pbf --nkv keyValueList="amenity.school" --wx schools.osm

rem use osmconvert to get osm file
osmconvert sample_osmosis.osm.pbf -o=sample_osmosis.osm

rem use osmfilter to get result
osmfilter sample_osmosis.osm --keep="amenity=school" -o=schools2.osm

Open a text edit to search tag <tag k="amenity" v="school"/>

Tools	Filename	Tag count
-	sample_osmosis.osm	78
osmosis	schools.osm	55
osmfilter	schools2.osm	78

I'm not sure the reason why this happened. Any idea?