Odd Shares / Sell Question

I have a question I suddenly wanted to ask and apologies if its worded not well. Ive always owned shares in their entirety and sold the whole bunch when I was finally done with a stock. However today it occured to me I might not want to do that with MSB. So here is kind of my thinking and Im hope somebody can correct me as I feel I must be wrong here.


Lets say you have 15,000 MSB shares and you own them at $4.00
Then you bought another 15,000 MSB shares at $1.60 and without being accurate about it lets say you now own 30,000 MSB shares at $3.00

If you then sold those addittional 15,000 shares at 2.10 for some profit (because you bought them at 1.60) and then purchased ANOTHER 15,000 MSB shares this would lower your purchase price of the overall 30,000 even further down from 3.00 to maybe 2.40 or something.

But this to me seems............not correct? In terms of simple calculations and average down or up calculators its right. But it doesn't make sense to me.

I still feel like the original 15,000 was bought at 4 and its only worth $3.00 for purchase price if you continue to hold the addittional extra 15,000 you bought. But then selling that addittional 15K shares and then buying more shouldn't lower your average purcahse price again......

Yet on share trading platforms it would because once the addittional 15K was sold its not like your holdings revert back to the original $4 purchase price. The platform would still think you have the purchase at 3.00

Sorry its a twisty and convuluted post but I hope people who do this a lot get what I'm driving at.