A5 is exactly 148×210 mm and the customer has confirmed that there is nothing to fix.
Adobe InDesign Team
An error occurred while saving the commentPeter Villevoye commented
For the ISO standard, the DIN institute has rounded off the exact sizes of the A-series to whole mm values. So going from A4 to A5, the rounding-off flips, and cuts the smaller half of the mm. (Hence the 297 mm for A4 is actually a bit too large...)https://en.wikipedia.org/wiki/ISO_216