Is this normal for Case fans?

69ingChipmunkzz

Honorable
Jan 9, 2013
167
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10,690


Is it okay that the rear one is spinning way way faster than the front? wont this mess up the airflow? The one that is spinning faster (i tihnk is the rear fan) is always about 10 or 20c more than the others- this was using Open Hardware monitor which gave me different readings.
http://gyazo.com/ca56b6fb90dca964e14ca41fda5bb4fc