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	Messier 106 - this is an extreme stretch of the r-band stack showing problems on the scale of individual ccds.

Why is my photometry from images so different from the catalog photometry?

The pixel values in PS1 images must be transformed using a non-linear equation to convert them to fluxes before doing photometry on them.  If you do not apply that transform, the photometry results will not be remotely close to the correct photometry. (Note that this transformation is required for full skycell images; smaller cutout images have already been converted to linear fluxes.)

Even after applying this non-linear transformation, photometry from the images will not match the catalog perfectly. The ubercal procedure was used to modify the photometric calibration using sources observed in multiple images.  That allowed correcting small errors in the photometry that depended on the exposure conditions, the epoch of the observation, the position in the detector, etc. Since this calibration was completed after the creation of the images, the image FITS headers and flux scaling do not reflect the improvements.  Measurements made directly from the images will consequently not be as accurate photometrically (or astrometrically) as measurements in the catalog.

Flux differences that originate from post-image calibration should be small, however.  Gross differences in photometry are much more likely to result from not applying the non-linear flux transformation.