a. Excel treats them as numbers instead of strings of digits and thus drops the leading 0
b. Developers make assumptions about postal codes based on how they work (or more usually how the developer incorrectly thinks they work) in their own country and these assumptions absolutely do NOT hold in other countries.
A relevant guide to geocoding and postal codes: https://opencagedata.com/guides/how-to-think-about-postcodes...
Until very recently I naively assumed that the area of a given zip code would be entirely within the area of some single city or town which would then be entirely within the area of a single county.
It was quite a rude awakening working with software that tries to apply the correct local taxes to a given address and finding that the statement “A given X can contain multiple Y” is true for every possible combination of zip, city, and county.