Please ensure that you have tried changing the "prayer calculation method" and the "high latitude method". These methods are the primary reason that time differences occurs in Fajr and Isha timings.
Please note that not all prayer times web sites use the same formula and the same set of parameters. This will give different results and this in turn will mean that prayer times will be different when comparing the 2 sources. This will happen whether the source is another website or whether its a timetable from your local mosque.
Prayer times are calculated from a very complex calculation and formula, involving amongst other factors the latitude and longitude of a particular location. As a result, variations in times can occur by different organisations according to approximations made using these calculations. Furthermore prayer times in high latitude areas differ even more so and this is more apparent in the summer months. As a result of this, it becomes more difficult to determine Isha and Fajr times. Hence an approximation is required and this is the reason the high latitude method was introduced.