The T-score calculation on Stages Link is functionally very similar to TSS, but differences in the processing of the file can result in a slightly different number. The number can be slightly different based on a number of reasons, such as data recording settings, difference in threshold programmed into your head unit, spikes which are filtered out (based on discard settings set in your profile), etc. By using our own data processing rules, we can ensure that the calculation is done consistently in order to give reliable data to plan your training and less affected by the settings of individual head units.
To minimize the difference between the values, we recommend setting your Garmin device to include zeros in the data averaging (under Data Recording settings) and to ensure that your Garmin setting for threshold power is identical to the setting in your Stages Link account. This will minimize any differences between the two so that you can receive reliable numbers while riding that will match up with the post ride analysis.