That optimistic 2.5% depends on who is doing the counting & when.
Of course, 2.5% is still by all means, far better over when I began running Linux over 9 years ago (late 2008). As to the numbers, these are often fudged, most Linux users could care less about market share, as long as we know we're a growing group. Part of the proof is the sheer number of Linux based material or instructions on the Internet compared to 9 years back, when these were rare & often didn't address what the masses needed, usually power users.
About fudged numbers, Apple, Google & Microsoft keeps up with these counters, especially the latter, all that have to do in instruct all resellers to have every demo computer connected to the Internet with screen saver lock. Of course this costs, yet am sure that the retailers gets something for their efforts, be it some free product (extra computers) or other means of a kickback. Once all of these computers stays connected, the stat counters cannot tell the difference between those on demo & in actual use, so Microsoft can gain an additional 0.5-1% from that alone. Stat counters, while not totally useless, on the other hand, aren't an exact science. It's simply a calculated guess.
At the same time, most Linux users doesn't keep up with this & if so, most won't have their computers running just to be a part of these counters, home users has no incentive to keep ours running 24/7. If by rare chance, most were connected during a count, usage would show a lot higher. Some counts has shown Linux to be over 3% in 2017.
More proof that Linux is growing are the number of what was once primarily Windows tech forums now has a Linux section. While not as active as the latter, still every new member counts, many are new to Linux & needs to learn. Many also realizes that the clock is ticking down on the EOL date of Windows 7 (Microsoft will likely, as with XP, host a 'Windows 7 Countdown' to lure folks to 'get up to date'. Many of who has declined W10 when offered, stating a loud NO to their efforts, some going as far as altering the registry to disable OS upgrades & disabling Telemetry at the same time.
Finally, while no OS is bulletproof (look at the recent dual threat Intel scandal that affects all OS's), more users are aware that Malware & Ransomware are real threats & here to stay. Take a peek at those in the 'Am I Infected' line, and at best, maybe a couple of Linux users are there, usually a false alarm. While infections can travel through a Linux computer & possibly be passed onto others, particularly on the same network, the chances of actual infection are slim.
Given all of the above, I feel that Linux usage is higher than 2.5%, there's lots of reasons to believe this number is on the conservative side.