Different number of nodes (schools) in OSM data in shp- and pbf-fomat

Dear OSM-experts,

I am quite new to OSM, and trying to use its data for my research topic.
First of all: thanks for making this possible!

I just stumble over an issue, which I cannot explain myself:

I downloaded the osm data for the Karlsruhe district (Regierungsbezirk Karlsruhe, NUTS2) at Geofabrik in .shp.zip and .osm.pbf format.

When I unpacked the .shp.zip-file and opened the file gis_osm_buildings_a_free_1.shp in arcMaps. Filtering for schools in the attribute table I found 1450 polygons.

After filtering the .osm.pbf.file for schools with Osmosis (osmosis --rbf karlsruhe-regbez-latest.osm.pbf --nkv keyValueList="amenity.school" --wx schools.osm), I opened the resulting file in JOSM. The file contained only 301 nodes and a school in my neighborhood, which is included in the shp ist oviously missing.

Can somebody explain, where this discrepancx might come from?

Thanks
Fester