I am no expert but when you attach a depreciation book to a fixed asset you are telling the system when to start the depreciation, the fact your purchase invoice comes in later is not relevant, you have the asset on site and start depreciating it from this point, not the point when the invoice comes in. If you wanted 335 days you should have set the depreciation start date to 01-May. If it ran from the purchase invoice the number of depreciation years and months would not work to enable straight line calcualtions etc. Why do you think it should work from the purchase invoice date?
the system always considers the depreciation start date, and not the posting date of purchase invoice, because for ex, you may purchase a fixed asset, but not put it in use until a later date.
for this reason, if you want to depreciate starting may 1st, change the depreciation start date.