Why does OD7000 not give me the actual distance to the target surface ?

OD7000 does not provide measurement values for the actual distance from the sensor head to the target surface.

Furthermore, although the specified working distances and measuring ranges are close to the actual distance from the sensor head, they are not exact.

This is primarily because the working distance of the sensor head always varies a little due to manufacturing tolerances, and for a device which can measure with nanometer resolutions, this is difficult to compensate for.
Instead the sensor works as a true displacement sensor and only provides measurement values within the specified measurement range.

For example, if I am using a device with the 6.2 mm ... 6.8 mm sensor head, the OD7000 will output measurements from 0 ... 600 μm.

You can see these values in Engineering Tool overview page and the alignment tool page.
If I am using the 35.5 mm … 39.5 mm sensor head, it will output measurements from 0 ... 4 mm.
And if I am using the 64 mm ... 74 mm head, it will output measurements from 0 ... 10 mm.

If a customer needs to know the actual distance from the sensor head to the target, we can only suggest that the customer position the sensor head with the target surface somewhere within the measurement range, then physically measure the distance to the sensor head using a physical measuring tool or gauge blocks and shims. 

If the customer then performs an initial measurement to the surface and records this as a zero point, subsequent measurements will provide deviation values and these can be used to calculate the resulting actual distance to the target surface.

Please note that as yet there is no zero set point function in the firmware.

6.2 ... 6.8 mm 

35.5 mm … 39.5 mm

64 mm ... 74 mm