Arduino interface with I2C Pressure Sensor
What's wrong?

Sensor is differential, up to 100psi. 
so \$P_{min}\$ is -100psi, \$P_{max}\$ is +100psi.
Total 14 bit (from 0 to 214-1), 
\$P_{min}\$ is at 10%, so \$OUTPUT_{min}\$ is 1638
\$P_{max}\$ is at 90%, so \$OUTPUT_{max}\$ is 14745

from datasheet:
$$ P=\dfrac{(OUTPUT - OUTPUT_{min}) \cdot (P_{max} - P_{min})} {OUTPUT_{max} - OUTPUT_{min}} + OUTPUT_{min}$$

 your first \$OUTPUT\$ value is  \$(31 \cdot 256) + 246 = 8182\$
according to datasheet pressure is:
$$\dfrac{(8182 - 1638) \cdot (100 - (-100))}{14745 - 1638} - 100 \approx -0.14496071$$
your second value is  \$(34 \cdot 256) + 102 = 8806\$
$$ \dfrac{(8806 - 1638) \cdot (100 - (-100))} {14745 - 1638} - 100 \approx 9.376669$$
precision is not so good, but it is still acceptable
third value: \$(32 \cdot 256) + 30 = 8222\$
$$\dfrac{(8222 - 1638) \cdot (100 - (-100))} {14745 - 1638} - 100 \approx 0.46540017$$
something goes wrong, it isn't like 20psi, it is like 0psi.

fourth value: \$(39 \cdot 256) + 167 = 10151\$
$$ \dfrac{(10151 - 1638) \cdot (100 - (-100))} {14745 - 1638} - 100 \approx 29.900053$$