To the average consumer, there is no logic, but to a marketing team there is, because it all comes down to what the consumer will bear or put up with.
On the product you have quoted, you are correct in the different price changes, because it doesn't take al that long for a 'savvy' shopper to find the best price on the day. One thing that I do notice on a very regular basis, is that the manufacturer cannot or will not compete with retailer's on their own products. And some people consider that going direct to the manufacturer offers the best product and service, when in reality it doesn't.
Perhaps going further with this issue, you don't need to look further that the USA and see how some of their prices for the same product are very different to here in the UK. We often hear that the relevant bodies are looking into this, but so far, very little evidence of price equality seems to have taken place.