Why there is a difference in between Absorbance and transmittance values in UV-Vis-NIR spectra in the same material?
Are you asking about, a difference between absorbance measured directly and absorbance calculated from measured transmittance?
Please make a clear question.
Absorbance is amount of light absorbed by the material.
Transmittance is amount of light passing through the material.
In oder to make differences in transmission values T a bit more clearer, especially in the low transmittance regime, a log scale is used.
The absorbance A is defined as:
A = -log10(T) with T= I/Io and Io = incoming photon flux; I= transmitted photon flux.
So at any event/occasion when the transmission T (also called transmittance) has gone down a factor of 10 in the linear scale, then the absorbance is increased by 1.
Nevertheless although absorbance A and transmittance T in general have different numerical values, they describe the same physical quantity (i.e. I/Io), but are presented in different scales according to the above equation.
https://en.wikipedia.org/wiki/Absorbance
If you tave taken those values (T or A ) from 'same materials' but different samples then it is a case of measurement uncertainty/precision and difference in individual sample composition, sample geometry, sample preparation, porosity, surface conditions...etc.
This might be the case even at different positions of the same sample.
Your sample thickness might be too large (very low transmitted photon flux), so detection precision of the transmitted beam severely enters the calculation of T and A.
In some cases, values T also depend on the angle of incidence of the beam on the material surface during the measurement.
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