Our meters are within +/-15% accuracy.
I am not sure if this means each reading is out by the same amount in the same direction or that every single reading could be out by a different amount. I suspect there is some variance with test strips on the same meter.
I mention this because, at higher levels, 15% is quite a lot.
Looking at the readings you have, if the "true value" was 14mmol/l, your before and after shower readings are pretty close to 14mmol/ - 15% and 14mmol/l + 15%.
In other words, within the tolerance of most meters, there may not have been much change in your BG.