Volume Equation

[PF]

I was testing this software with the following data of the amount of gallons per inch in a 5000 gallon oil tank.  This appears to be a simple test however the results were as simplified or accurate as I would of anticipated based on the software's ability to generate more simplified and accurate results in other data sets.  Would someone be able to explain why that would be?

Current Gallons	Current Inches	Total Gallons	Total Inches
cg	ci	tg	ti
14	1	5000	72
41	2	5000	72
74	3	5000	72
113	4	5000	72
156	5	5000	72
203	6	5000	72
255	7	5000	72
309	8	5000	72
367	9	5000	72
427	10	5000	72
489	11	5000	72
554	12	5000	72
622	13	5000	72
691	14	5000	72
762	15	5000	72
835	16	5000	72
910	17	5000	72
985	18	5000	72
1063	19	5000	72
1141	20	5000	72
1221	21	5000	72
1302	22	5000	72
1384	23	5000	72
1466	24	5000	72
1550	25	5000	72
1634	26	5000	72
1719	27	5000	72
1804	28	5000	72
1890	29	5000	72
1976	30	5000	72
2062	31	5000	72
2149	32	5000	72
2236	33	5000	72
2324	34	5000	72
2411	35	5000	72
2499	36	5000	72
2586	37	5000	72
2673	38	5000	72
2759	39	5000	72
2846	40	5000	72
2932	41	5000	72
3018	42	5000	72
3103	43	5000	72
3188	44	5000	72
3272	45	5000	72
3356	46	5000	72
3438	47	5000	72
3520	48	5000	72
3601	49	5000	72
3692	50	5000	72
3759	51	5000	72
3836	52	5000	72
3912	53	5000	72
3986	54	5000	72
4059	55	5000	72
4130	56	5000	72
4199	57	5000	72
4267	58	5000	72
4332	59	5000	72
4394	60	5000	72
4454	61	5000	72
4512	62	5000	72
4569	63	5000	72
4618	64	5000	72
4665	65	5000	72
4709	66	5000	72
4748	67	5000	72
4781	68	5000	72
4808	69	5000	72
4822	70	5000	72
4835	71	5000	72