The patent expiry usually varies by a day or two among the EU countries. For eg. SPC of EP0914118 will expire on different days in France, UK and Germany & Spain? Why is there such a difference when the filing date is the same? A similar observation has been recorded for other patents expiring a day earlier in the UK where there is no associated SPC.
Different countries count the life of a patent differently, and sometimes a patent expires in one country one or few days earlier than in other countries. This depends on the national patent law of each country, which may establish that a patent expires at the exact same date as the filing date plus 20 years, or the day before the exact same date as the filing date plus 20 years. So even if the filing date is common, the expiring date is not. Besides, I do not know how it works exactly, but there are some countries that also take into account whether the application was filed in a non-working day (e.g. a Sunday) for computing the expiration date of patents; pharmaceutical companies know this very well and take advantage of these things in order to maximize the protection of their compounds, because one or few more days of protection may result in large sums of money.
Since SPCs become effective right after the expiration of the patent, different patent expiration dates result in different SPC expiration dates.