Just another efficiency question

I started brewing with the grainfather about a year ago and have done 5+ brews. I have been milling my own grain from the start. All of the brews have had a problem with astringency: some more some less. So in my last brew I changed a number of parameters:

• I got the grain pre-ground from the shop to rule out that I was milling too fine
• I did not wait until the very last drop of the sparge but removed the grain basket which there was a little dropping still going on. I let the basket drip into a bucket after this and around 1 litre of fluid was in there
• I set the temp of the boil to 98degC as I live at 500m+ (had nothing to do with astringency was just a coincidence)

The grind looked good, there was a bit of flour and all grain were cracked or split. The sparge went noticeably better and quicker than with my own grind but it was still a healthy 40min. The boil was not very vigorous: it was not hopping like a volcano but there was a very good circulation going on.

It was this https://byo.com/article/neipa-style-profile/ recipe but changed a bit - I went with 5kg of pale ale malt - My boil was 60min - I did a hop stand but just added the hops at 0min and left to stand for 30 mins

I did not measure the pre-boil gravity or the volume. I think the volume was a few litres more than it should be (but not many). However I took a reading after racking and the gravity (at 20degc) was 1.048. This is a whole 13 - 15 points less than it should be.

I know it will probably turn out ok if a little weak and I am not obsessed with OG but I would like to hear some thoughts about what it could have been. I don't think the extra litres could have had such a large effect and like I said the grain looked ok. Did it have something to do with the oats or wheat in the recipe?