When I use an A/C, I use an oscillating fan in front of it to help project the cold air around the room. I also aim this fan at the ceiling, since cold air falls and because the cold air flowing over my hot hoods helps drop the ambient temps significantly. Having cold air blow at only one point and level in your room will not be as effective as spreading it around, and you will find hot and cold spots.

I was confused by your first sentence, as I am unclear whether you are saying that installing the A/C raised temps by 10C or if the summer weather is raising temps by 10C. I am assuming the latter. Remember, if the A/C is too small, you actually might be raising temps in the room because of the heat the A/C might have to put out to produce such cold air in such a warm room. It is always better to use an A/C that is atleast 30% better then what you think you will ever ever need. This will help the duty cycle, power consumption (it wont need to run as much), and keep a more consistent and solid temperature.

Your thermometer might only sample the temperature every few seconds or if it notices a constantly fluctuating temp...then maybe it is just giving you the average or something. What type of thermometer is it? Is it a regular cheap hygrometer? If so, they are very unreliable and can read nearly 10 degrees off.

That all being said, if everything looks great and your experiencing nothing bad...then I wouldn't worry about it. You will be able to tell if there is too much heat, as growth will slow, tips will burn, leaves will lose their moisture too quickly (especially if humidity is extra low, and remember an A/C is just a dehumidifier that blows away the hot air it generates so it will lower humidity).