Uneven surface finish on steel

Hi,

I was facing a part. But, the surface finish of the part is uneven, only one small part has mirror-like finish and feels 'flat', the other has tool marks. Can anyone explain what could've possibly gone wrong? I'm pretty sure that the part is clamped down hard enough. Hasn't check runout and the inserts have different heights, I used the lowest insert to setup for my z-axis.

material: mild steel
tool: 50mm shell mill with 4 AMPT1604 inserts at 90 degree
speed: 382rpm
feed: 76mm/min
DOC: 0.15mm
stepover: 75% of tool diameter
coolant: blast

Thanks.