Why does User Activity Report cuts in the middle off when exported to excel

Why does User Activity Report cuts in the middle off when exported to excel

I just generated a User Activity Report, under Reports heading (nothing custom or fancy, Just a canned report). When I export to excel it cuts off around 350 user records, while I have 550+ users.
Can anyone tell me why this is happening ? Exporting to PDF works fine, but I need the output in excel format so that I can sort and manipulate according to what my management needs.

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