Welding gas quality

Are there different grades of mig welding gas or have I been getting ripped. I use 75% argon 25% carbon dioxide.
I've been buying my gas at the same supplier for about 15 years mainly because it's the only one around, their prices aren't good and have heard alot of people grumbling, one shop I worked for noticed they had been overcharging his account for years ,they gave him a free bottle of gas, wow
Last week a buddy wanted to use my welder to plate his frame. he came over and started welding, got one half way on and ran out of gas. He suggested taking the bottle to a new welding supply that had just opened nearby. I was pleased to see they were about six bucks cheaper. We simply removed one bottle and replace it with the full bottle changing nothing else. He got back under the jeep and started welding. I instantly noticed the sound of the welder was very different, a smooth fine frying sound. He welded about 4 beads climbed out from under the jeep and ask if I had changed the setting because it was welding better than it had ever but I hadn't. The four beads he had welded were NICE, very clean and blue gray in color with copper spots from the wire coating, the other welds from earlier were good welds with good penetration but were more sooty with spatter.
What do you think happened? Over the years I've had different welders, tried different wire guages, heat and alloy and always felt it could be better and chaulked it up to not having a 3 grand welder.
Just thought I'd post this, might help someone

An interesting observation. I mean, how would we ordinary folk know if there's really argon in that mix? On the other hand, you should also remember that if you were using a single-stage regulator, and not riding the control constantly, it would perform quite badly as you get to the bottom of the tank, so a new tank would make an immediately tangible difference.

Matthew Currie said:

An interesting observation. I mean, how would we ordinary folk know if there's really argon in that mix? On the other hand, you should also remember that if you were using a single-stage regulator, and not riding the control constantly, it would perform quite badly as you get to the bottom of the tank, so a new tank would make an immediately tangible difference.

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I see what your saying and normally I'd agree but there's too big of a difference, I've been welding with that welder over a year now and it's never been that smooth. Talked to a friend last night that uses straight carbon dioxide in his mig, he says it's alot cheaper than argon mix but has a dirtier weld. he welds a lot of large stuff and says the dirtier welds doesn't matter to him. No way to prove it but I honestly believe their mix is off and putting in more of the cheaper carbon dioxide and little argon, does anyone check this or are you at their mercy?