structure queries

According to the ann of EPE on 28/9:
"Nulla Nulla-1 is programmed to a total depth (TD) of 2420 metres and will take
approximately 24 days to drill. The primary objective, the Hutton Sandstone is expected
at about 1820 metres."

Presently at "Depth 1852 metres
Current Operation Drilling ahead in the Birkhead Formation
Forward Operation Drill ahead to the Hutton Sandstone
Spud Date 14 th October 2004
Programmed total depth 2420 metres
Oil shows commenced at 1812 metres and have persisted down to the current depth. These
shows are manifested by up to 100% but predominantly 40% to 80% bright yellow to dull orange
patchy to pin-point fluorescence with a streaming cut and a thin residue ring. Associated with
this fluorescence are elevated gas values of 2 to 4 times the background value with the gas
composed of C1 to C5 components.
The interval of shows consists primarily of claystone and siltstone with only minor fine grained
sandstone.
Correlation with the MacKillop # 1 well located 16 km to the east indicates that one of the
primary targets, the Hutton Sandstone, should be intersected at about 1880 metres.'

How is it that they are so far out? I may be mistaken but how can you be out by 80 meters and feel confidence in actually hitting a targetted reserviour if your data is so variable

Could a more experienced oiler comment...and is this the norm?