What IS my (upload) speed anyway? (Not a problem, just a question.)

So, I went through some fairly ugly times before declaring (with some assistance here) that my Motorola MB8611 just was not interoperating with the network consistently (the post: "One last try with MB8611, then what?").  I did some research and ended up purchasing an Arris S34 modem, which is performing very well.  

I'm in a mid-high/split ("Next Gen Speed Tier") area.  After I upgraded my service, with the new modem, I was finding my upload speeds were very acceptable, at ~130Mb/sec.  Note that I'm rather paranoid about backups, so I vault to AWS S3 hence my being more aware of upload speeds.  

So, it was even nicer last month when I noticed that somehow, my speed was increased to somewhere around 375 Mb/sec.  

I'm curious about why it's now been reset to around 175Mb/sec.  Here's a graph:

Yes, on some level (for me) higher is better, at least outbound.  I'm not complaining, and I appreciate even the still-enhanced speeds I'm seeing (I think this is higher than what I understand is the spec for my level of service), but I'm wondering if anyone has any explanations or observations... 

Thanks!