Invoice discount shows in pdf in recurring invoices although value is 0

Recurring invoices always show an invoice discount although the amount as well as the percentage are empty.
In the default template (line 201 and line211) i see a comparision against the value != '0.00’
In normal invoices this seems to work but suddenly a recurring invoice shows the discount.

I did change the comparison to > 0 and this seems to work for me but still i am wondering what the reason for this behaveour is.

Below you see both lines after the change:

<?php if ($invoice->invoice_discount_percent > 0) : ?>

<?php if ($invoice->invoice_discount_amount > 0) : ?>