I have the same exact values on two different graphs. Why when I put a linear and exponential trend line (one on each graph) do the formulas become different. For example, the starting values change

T (mins) | 0 | 5 | 10 | 15 | 20 | 25 | 30 |

A (°C) | 72 | 59.7 | 54.3 | 47.9 | 36.2 | 34.2 | 32.8 |

Exponential model

A = a x b^t

A=70.11 × **0.97t**

a = Starting value. The coffee temperature at zero minutes was **70.1 **degrees celsius

Linear model

A = a(t) + b

A = -1.3351t + **68.171**

b= y-intercept. The starting temperature of the cup of coffee at zero minutes was **68.17** degrees celsius