Hey Google, what's up with your search bar?

Why does Google change the search bar on Nexus/Google experience devices all the time?

Just take a look:

Froyo style search bar

Nexus-one-e1324576259330_medium

via oyster.ignimgs.com

Oppa Gingerstyle

Google-nexus-s-android-gingerbread-phone_medium

via www.cellphones.ca

Honeycomb

Xoom-android-ui-03_medium

via www.ubergizmo.com

Ice cream sandwich

Nexus-prime-android-ice-cream-sandwich_medium

via www.smartkeitai.com

Jelly bean 4.1.1

Nexus-main_medium

via www.cvr.cc

Jelly Bean 4.2/ Key Lime Pie 4.2

Nexusae0_user2804463_pic28572_13496449691_medium

via cdn.androidpolice.com

Why Google? Why?

Why does Google change that thing all the time?

And what do you think is the best search bar?