First off, nobody knows exactly what the numbers are, but you need something like 35,000 accumulative net transfers to trigger a price rise. So if a player receives +5,000 Net Transfers per day then after 7 days his price will rise. Julian Speroni's net transfers have been in the low thousands for a while, but he suffered a lot of low-thousand NTs out for a while, too. The recent NTs in in the past couple weeks should push him above the threshold now though - you might see his price rise in the next couple days. Price falls are much more inconsistent, but it follows a similar principle.
The value of a player can only rise or fall by £0.1m per day. So if Mertesacker gets -70,000 net transfers out in one day, he'll lose £0.1m on that day, but if he gains +35,000 NTs each day for the next two days, then he'll gain £0.2m, leaving him £0.1m higher than he was before, even though the overall Net Transfers in that Game Week was 0.
Further compounding this is that a player's value can only change 3 times per week, so if there are rumours on Monday that RVP is injured again and won't play, maybe 40k people will see the news and transfer him out on Monday and his value drops by -0.1m. Some people might hold on to him as it's just a rumour. Similar thing happens Tuesday as the news spreads: another 40k people transfer him out, and his value drops by another -0.1m. Wednesday comes and as the news makes the rounds, even more people decide they want to transfer him out, this time 60k. Most people have still held on to him though, because at this stage, it's just a rumour, unconfirmed by Man Utd's press team. Now the total NTs is -140k in that GW, which is 4 lots of 35k, but since the price change is capped at 0.1m per day, RVP still only loses -0.1m. Additionally, accumulated NTs get reset after each price change, so the extra 5k of transfers in previous days don't get carried over.
Now, on Thursday, Man Utd definitively announce that RVP is out for at least 8 weeks -- it hits the back pages in big capital letters, is all over the internet, everyone's talking about it at work, etc etc -- so now everyone decides they're going to sell RVP, and he falls from ~35% to 15% in one day.
That's maybe -400,000 NTs out. 400,000 people spread over 12 days would equate to a price drop of -£1.1m. However, 400,000 people in 1 day is capped at -£0.1m. And further, because there is a weekly cap of only 3 changes per week that has already been triggered, these -400,000 NTs out won't affect RVP's price at all. RVP stays at -0.3m lower than where he was at the start of the week, even though hundreds of thousands of people have transferred him out.
Basically, the price changes are not obviously related to either performance or net transfers, nor to injuries or real life transfers.