Is that the max you've ever seen? Is that what some calculator told you? Age/weight/fitness makes a huge difference. Your "max" won't necessarily change, but you should see your HR come down for the same effort over time. With that said, you could be more fit and more willing to push harder, which would result in you seeing higher numbers overall.

It could go up because of a million variables - Heat, wind, salt, dehydration, lack of sleep (or anything that increases adrenaline or cortisol). 3 bpm over 170 is very small. When I'm pushing threshold on intervals I'm regularly up in the 170-180 range, sometimes higher. This is fine, and for while working out is perfectly healthy, assuming you are generally healthy enough to handle this strain. The max I've ever registered was 210 when I absolutely exploded trying to chase down the paceline on a fast group ride when I was spit out the back. That was not fun at all. 170 is normal for a good effort depending on your age and conditioning.