Can anyone tell me why I didn't get frog spawn when the last frog was spotted.

You’ve attracted 91.  Every 5th lays spawn so you’ll need to attract 4 more.

That last frog was your 91st. and it's every 5th. that drops spawn

I was told that I had to include the ghostly frogs attracted in my total, I'm guessing that isn't the case then

I don't know, but i have never counted those and been ok. As long as you're due for frogs.....by having froglets attracted then every 5th. drops spawn.....

You add the ghostly ones to the underwater frogs attracted.  It may not be an issue if you have plenty of tadpoles or froglets in hand, but if your numbers are closer and you didn't count the ghostly frogs, you could find you think you're due a frog when you're not.

Every five of either does drop spawn, if you're due the next fifth.  But if you had, for instance, 64 underwater frogs and 65 tadpoles and froglets you would  need more frog to drop spawn - but if you had also attracted one ghostly frog, the numbers would be 65 overall so you wouldn't be due another frog.  The 65th would still drop spawn, but you'd need more froglets in order to get to that 65th.

You count the ghostly frogs with the adult underwater frogs to know if you have frogs due, but for spawn they are still separate numbers.