2.1.702 Part 1 Section 18.8.21, fills (Fills)

a.   The standard states that the fill child element has a minOccurs value of 0 and a maxOccurs value of unbounded.

Excel limits the occurrences of this element to be at least 1 and at most 65430.

b.   The standard allows for arbitrary definitions of these elements

In Excel, the first two Fill ("[ISO/IEC-29500-1] §18.8.20; fill") values are reserved and deviations from their predefined values are not respected