Issue with calibration : unwanted transverse displacement while recording a pure vertical motion.

[Sly0650]

Dear community

I have an issue at the end of the calibration stage of a single camera system.
After the calibration with the target, I record a non-deformed speckle pattern being moved vertically (attached to the tensile machine crosshead).
Although the motion of the speckle is purely vertical (or at least unidirectional), a transverse (horizontal) deviation of approx.. 0,150mm is recorded for an actual displacement of 10mm vertically (accurately measured by the machine). Vertically, the result of pattern displacement is also 10mm with very small variation around the nominal 10mm, depending on the point on the pattern.
The calibration step with the calibrated target gives 0,13.
This transverse deviation has been repeatedly measured with several calibrations and several displacement tests. For some other trials, the transverse could be even bigger than 0,150mm.
Any idea where the transverse deviation could come from ?
Additional question : how the X-Y axis system after calibration (XX_displacement and YY displacement) compares with the "vertical" axis defined by the displacement of the crosshead of the machine ?

Our equipement :
Camera : pi caméra V3
Résolution : 1920x1080
Focus : fixed
Speckle size : dot 0,33 and 0,66mm

Target : 7x6, black dot, spacing 6,5mm and offset 1

Thanks in advance for any valuable information / suggestion