I know what percentage represents, but what you fail to understand is that 45 % is still close to 55 % regardless of the quantity it repsents, that's what percent are used for, not to represent an actual number quantity.
1 % can represent a small or humongous quantity, but it's undeniable that it also represents 1/100.
Hence 45 % can represent as you have said 45 or 45 * 10 ^6, but the truth is still that 45 % is close to 55 % regardless of the actual difference between them, it's the percentage difference that matters.
If during all this generation X1 maintains close to 45 % marketshare, nobody can claim that PS4 dominated US, it will always remain a close battle.
No, the actual numbers are what matters, not the percentage. When reports of a system getting an X% boost in sales, people are rightly skeptical of the figure because it can mean very different things. Despite a high percentage, the initial sales of said product could be low to the point that the increased amount of sales is small. Same thing applies to marketshare.
Let's say 3 years later, PS4 is at 55 million and X1 is at 45 million. That's a 10 million gap. The amount of time for the X1 to overcome that gap will take longer than overcoming a 600k gap. Let's say MS wants to overcome that 10 million gap in a year. They need to outsell the PS4 by 833,333 units a month versus 50,000 a month if they want to outsell the PS4 if the gap is 600k. That is a 1666.66% difference!!
PS4 is growing its lead and it's impossible for the X1 to catch up. PS4 has the advantage in terms of specs, networking, reputation, multiplats, exclusives, genre diversity, and value. For you to deny this is absolutely silly.