To clarify, are you saying that your 3 year old 2015 laptop has, today, better battery life than your new 2018 laptop? That's mad...
I'd guesstimate I get at most 3 hours on the 2017 before the battery level gets low enough I have to start planning where I'll next plug in. Not ideal.
I suspect that Apple will be seeing a lot of 2015 MBPs brought in for battery replacements...
physics might have a word with you :)
When it was new i would regularly get ~12 hours of light usage from it (safari, email, terminal, VPN, citrix, remote desktop). It has been providing around 8-9 hours steadily for the past year or so. I finally noticed when placed flat on a table it wasn't sitting flat, and the battery was swollen.
Now with a new battery we're back to 12+ hours of light usage.
Nowadays, you can't even get a 15 inch MacBook Pro without a discreet GPU.