Why speed variances on Fibre Pro package?

[#1]

Some of you may have seen my previous post on speed and have slightly different question/observation - so apols for the new post
Been monitoring the speeds over the last couple of days and can't understand why it varies so much.  Ping 5-25ms, download 17-30mb.  These changes can be seen with only an hour gap between tests.  Thought on the Extra +Pro package I wouldn't see this type of fluctuation.  Certainly didn't when I had dsl on Pro.  Not really sure it's worth the £25 pm premium if this is now considered normal.
Router is 2 feet from modem
PC is connected to router via homeplugs
Profile is 37mb
Are speed changes like this normal?  Only thing that has recently changed is the modem being replaced by plusnet.

[#2]

I could not understand why my desktop, connected via Solwise PL-85PR (85 Mbps it says on the tin)  recorded 20 Mbps but my wife's laptop which had been used to test the connection was 37 Mbps until I connected it with a cable. Replaced the homeplugs with D-Link Powerline AV - Bingo!
Try a cable connection.

[#3]

Not a bad idea, will try a cable for comparison. 
I did this when I first got fibre and my current plugs though (Devolo 200 AV Plus) and speed was 37mb.  But I suppose things may have broken since.

[#4]

Hi Discord,
what was the speed with ethernet?
Jojo